I am sure that what was wrong was my lack of size. I just don't think I had enough to satisfy her. Certainly, she was not too pleased. I finished after about 30 seconds, and then I asked her how it was for her.

She replied: 'Not very good.' Then she got up and walked off. I have not seen her again since. I am certain that she must be telling all her friends that I am too small to make a girl happy.

Doc, I have measured myself carefully, and I feel that I am tiny. My length seems to be just over six and a quarter inches when I am fully erect. I have not seen any other guys erect, but it is clear that they are much bigger than me.

One of my friends told me he was nine inches long, and another one said he had 10 inches. They were laughing at me.

So clearly, I am under-size. Could I take an operation to cure this, doc? Or should I try and buy one of those things that they advertise on the Net that give a guy extra length and power?

A. You may be surprised to hear that I get many emails and letters like yours. There are large numbers of young men who fret constantly about their length. But in most cases, they have nothing at all to worry about.

Your friends are exaggerating.

You say that you are 'just over six and a quarter inches long when fully and totally erect'. So, let us call that 6.25 inches.

Now, let me acquaint you with a few medical facts.

Over the years, there have been three major scientific surveys of penis length. The first was carried out by the great United States sexologist Dr Alfred Kinsey (of 'Kinsey Report' fame) in the late 1940s. He studied 3,500 young males, and he found that their average length when erect was just 6.2 inches - or so they claimed. You will note that this is slightly less than yours!

A second survey was carried out more recently by the world-famous condom manufacturers, London Rubber International. They studied almost 3,000 men from all around the world - though I believe the majority were American. Again, the men measured themselves. The result was that the average penile length is 6.4 inches. And this is only a fraction of an inch above your own measurement.

Finally, there was another major survey conducted by Lifestyles Inc, which is another condom manufacturer. For various reasons, this one was rather more scientific than the other two studies. Most important, the guys did NOT measure themselves. Instead, an independent observer did the measuring.

And the result of the Lifestyles survey was that the average male has a length of just 5.87 inches. This seems to me the most believable of the three surveys.

However, I can sum the surveys up by saying that the average guy has an erection that measures about six inches. Yours apparently measures 6.25.

So it is obvious that you are NOT small at all! You are normal.

OK, then why was the girl dissatisfied with you? Why did she walk away?

There may have been all sorts of reasons. But what strikes me most forcibly is that you finished after only 30 seconds. These days most females expect their partners to continue with intercourse for a reasonable amount of time. Ten minutes is quite common, and some successful guys can manage half an hour.

But 30 seconds suggests that you are climaxing far too soon. It may even be that you have that premature ejaculation thing. I shall return to that subject in a future column.

A further point is that it sounds like you did not offer this young lady any 'love play', or use manual/oral techniques which would give her pleasure. I am not criticising you for this, but it is obvious that you have a lot to learn about sex and about what pleases women.

A. This is called stress incontinence. You may well be able to cure it by doing special pelvic floor exercises to tighten up the muscles near the bladder. Any midwife or nurse could show you how to practise these.

A. No. For some reason, your puberty has been delayed. Please ask a doc to send you to a gland specialist (an endocrinologist), who will do tests and probably give you hormone treatment.

A. Yes, it is possible that there is some membrane or other structure blocking the access to your vagina.

However, it could also be that you have vaginismus, which is a sort of 'muscle contract' that occurs when young women are too tense.

Please see a doc this week. He or she will do a vaginal exam, and I am sure will be able to sort this out for you.

A. Afraid not. Parents often say that kind of thing, but in reality the eating of candy makes no difference to acne and spots.

I would suggest that you try three months of daily antibiotic pills, prescribed by a doctor. Also, the doc will probably suggest a good anti-acne preparation which you could apply to your skin. Good luck.

A. No, although it may make you a little tired next day. You will find that your need to discharge will diminish somewhat as you get older.

A. Certainly. However, I do think that you should have an internal check-up by a doc to see if there is any serious reason for all this bleeding.

However, the odds are that the Pill would enable you to have a period that only lasts three or four days, and which arrives on the predicted date every month. This would be much better for your health.