Objective: There is a lack of evidence to show the role of exercise intensity in the prevention of cancer mortality because no previous studies have shown this relation. The relationship of leisure-time physical activity with cancer mortality was therefore assessed. Methods: Participants were from a population-based sample of 2560 men from eastern Finland with no history of cancer at baseline. Physical activity was assessed using the 12-month leisure-time physical activity questionnaire. During an average follow-up of 16.7 years, a total of 181 cancer related deaths occurred. Results: An increase of 1.2 metabolic units (MET or metabolic equivalents of oxygen consumption; 1 SD in metabolic equivalents) in the mean intensity of leisuretime physical activity was related to a decrease (relative risk (RR) 0.85, 95% CI 0.72 to 0.99) in cancer mortality mainly due to lung and gastrointestinal cancers, after adjustment for age, examination year, alcohol consumption, smoking, body mass index and energy, fibre and fat intake. Men with leisure-time physical activity of more than 5.2 MET (highest quartile) had a lower (RR 0.63, 95% CI 0.40 to 0.99) cancer mortality compared with men whose mean intensity of physical activity was less than 3.7 MET (lowest quartile). The mean intensity of physical activity was related to the risk of cancer death among men who exercised at least 30 minutes per day on average. Conclusions: This prospective study indicates that the mean intensity of leisure-time physical activity is inversely associated with the risk of premature death from cancer in men.