Giovannucci et al1 found a lower risk of advanced and fatal prostate cancer in men 65 years and older, who are in the highest category of vigorous physical activity in the Health Professionals Follow-up Study after adjusting for a large number of covariates. The authors mention a risk reduction of 3 to 4 fold and suggest that a putative uncontrolled confounding factor would have to be strongly related to both high levels of physical activity and a substantially reduced risk of advanced prostate cancer.
Aydin ZD. Sun Exposure May Confound Physical Activity–Prostate Cancer Association. Arch Intern Med. 2005;165(21):2538–2539. doi:10.1001/archinte.165.21.2538-c