Regular smoking was linked to the most aggressive form
Frequent or long-term marijuana use may raise a man's risk of testicular cancer, American research suggests.
The study of 369 men, published in the journal Cancer, found being a regular marijuana user doubled the risk compared to those who never smoked it.
The results suggest that it may be linked to the most aggressive form of the cancer.
A spokesman for Cancer Research UK said that no previous studies had found a link between marijuana and the disease.
Testicular cancer is one of the most common cancers in younger men, with approximately 2,000 new cases each year in the UK.
Incidence in Europe and North America is far higher than in some other parts of the world, and has been rising steadily for no apparent reason.
Known risk factors for the cancer include previous injuries to the testicles, a family history of the disease, or suffering from undescended testicles as a young child.
The study from scientists at the Fred Hutchinson Cancer Research Center in Seattle is the first to look specifically at marijuana use in relation to the disease.
They studied 369 men aged 18 to 44, who had been diagnosed with testicular cancer, and quizzed them about marijuana use.
Their replies were compared to those from almost 1,000 apparently healthy control subjects.
Even after adjusting the figures to take account of the other known risk factors, marijuana use remained a clear risk factor for testicular cancer.
Just being a marijuana smoker seemed to carry a 70% extra risk, while those who smoked it regularly, or had smoked from an early age, had twice the risk compared to those who had never smoked it.
A connection was made to nonseminoma, a fast-growing form of testicular cancer which accounts for approximately 40% of all cases, and tends to strike younger.
Dr Janet Daling, one of the authors, said that puberty might be a "window of opportunity" during which boys were more vulnerable to environmental factors such as the chemicals in marijuana.
"This is consistent with the study's findings that the elevated risk of nonseminoma-type testicular cancer in particular was associated with marijuana use prior to 18," she said.
Testicular cancer is usually easily treated
Another research, Dr Stephen Schwartz, said: "What young men should know is first, we know very little about the long-term health consequences of marijuana smoking, especially heavy marijuana smoking, and second, our study provides some evidence that testicular cancer could be one adverse consequence."
The next step, he said, would be to look more closely at cells in the testicles to see if any of them had receptors set up to respond to cannabis chemicals.
Henry Scowcroft, from Cancer Research UK, said: "As the researchers themselves point out, this is the first inkling that there is any association between chronic marijuana use and testicular cancer.
"But the researchers only interviewed a relatively small number of men.
"So before we can reach any firm conclusions about whether this is a cause-and-effect relationship, rather than a statistical blip, the result needs to be replicated in a much larger study."