If a man wants to become a father, he should avoid long hot baths and stick to showers instead.
Scientists say they have confirmed the "old wives’ tale" that prolonged immersion in hot water affects male fertility.
Men used to spending half an hour or more in a bath or jacuzzi who gave up for three months were found to increase sperm production almost fivefold, according to research published in the journal of the Brazilian Society of Urology.
Sperm needs cool surroundings to develop best, which is why the testicles are outside the body.
Because of this, men who want to father a child are already advised not to wear tight clothing or underwear which causes too much heat to build up.
In a pilot study involving men with fertility problems, half of those who refrained from baths saw their sperm count rise by an average of 491 per cent.
The others failed to respond but they were all smokers. Tobacco has long been recognised as a major factor in infertility.
Dr Paul Turek, who led the team from the University of California at San Francisco, said: "It has been believed for decades that wet heat exposure is bad for fertility, but this effect has rarely been documented.
"We now have actual evidence to show patients that these recreational activities are a real risk factor for male infertility."