392 men had been diagnosed with prostate cancer, while the other equal number were healthy men.
The researchers found that obese men were not only more likely to die of prostrate cancer, but were also more likely to develop an aggressive form of the disease which was likely to spread to other parts of the body.
The findings were described by lead researcher Dr Par Stattin as 'provocative'.
"The suggestion that obese people are less likely to develop prostate cancer is provocative," the BBC quoted Dr Stattin, as saying.
"Possibly, low levels of testosterone, the male sex steroid that is low in obese men, may explain why these men are less likely to develop prostate cancer in the first place.
"But they may be at a higher risk of a more aggressive form of cancer that is less reliant on testosterone," he added. Dr Greg Martin, of the World Cancer Research Fund said that what the study highlighted that being overweight increases the risk of developing a number of different cancers.
"While this study suggests that obese people could be less likely to develop prostate cancer in the first place, it is important to remember that being overweight significantly increases your risk of developing a number of different cancers, and is bad for your overall cancer risk," he said.
Added Dr Chris Hiley, of the Prostate Cancer Charity: "Men should not get the impression that there is an up side to obesity because it looks as if it might prevent prostate cancer. This is not the story at all.
"Cancer and heart disease are the biggest causes of preventable deaths and getting your weight under control with a healthy diet and lifestyle cuts the risk of both."
Funded by the World Cancer Research Fund, the study appears in the International Journal of Cancer.