Men who start to lose their hair by age 20 - a syndrome known as pattern baldness - are twice as likely to develop prostate cancer later in life, according to a new study.
The findings, published this week in the Annals of Oncology, could help identify men who should be screened early and more often for disease, the researchers said.
Prostate cancer is the commonest non-skin cancer among men worldwide and, after lung tumours, is the second biggest cause of death from cancer among men in the United States and Europe. Most cases occur among men aged in their sixties.
Earlier research has shown that sex hormones called androgens play a key role in the development of both pattern baldness and cancer of the prostate, a walnut-sized gland near the bladder crucial to the male reproductive system.
But the link between the two remained obscure, with at least one study suggesting that premature baldness actually pointed to a reduced risk of cancer.
To probe further, a team of scientists led by Philippe Giraud of Georges Pompidou European Hospital in Paris asked 669 men - 338 of whom had a history of prostate cancer - how bald they were at ages 20, 30 and 40, using standardised images for reference.
Men who did not start to lose their hair until age 30 or 40 showed no increased risk compared to the control group of developing the dreaded disease.
But for those who had early-onset balding - a condition known to doctors as androgenic alopecia - at age 20, the risk doubled.
Giraud said balding men should not panic. "The fact that a (young) man is losing his hair does not mean that he will have cancer," he said by telephone.
He also cautioned that the results would need to be verified in follow up studies.
But the findings suggest that premature balding could become a useful marker to help doctors screen for the disease, he said.
"Current prostate screening protocols are very controversial because some worry that systematic screening at 50 years old - without taking other criteria into account - will lead to over-treatment," he said.
Many countries have routine screening programmes for men in their middle age.
One of the problems, however, is that the so-called PSA antigen test, now 20 years old, cannot distinguish between low-risk tumours and aggressive lesions that are often fatal.
Antigen levels can also fluctuate according to the individual and may be skewed by prostate inflammation.
One out of two men lose their hair, but of the 50 percent of men who go partially or totally bald, only 10 to 15 percent suffer from androgenic alopecia, Giraud said.
Another study published last year showed that finger patterns could also help identify which men should undergo regular screening.
Men whose index fingers are longer than their ring, or fourth, fingers run a significantly lower risk of prostate cancer, the study found.