This prospective one-year follow-up study compared the risks of knee injuries in various commuting and lifestyle activities as well as in recreational and competitive sports in a 15 to 74-year-old Finnish population cohort. A cohort of 3657 persons was randomly selected from the nationwide population register of Finland. Ninety-two percent of them accepted to participate (n = 3363). The subjects were interviewed by telephone three times during the one-year follow-up. The recorded data included all physical activities that lasted 15 minutes or more, and all injuries that were sustained during these activities. Fifteen percent (n = 321) of all reported injuries affected the knee. The individual risk of knee injury per 1000 exposure hours was low in commuting activities (cycling, walking), 0.06 (95 % CI 0.04 to 0.09) and in lifestyle activities (gardening, hunting, fishing, home repair etc.), 0.04 (0.03 - 0.06). In recreational and competitive sports, the knee injury risk was almost ten times higher, 0.44 (0.39 - 0.50). For commuting activities (p = 0.046) and for recreational and competitive sports (p < 0.001), there was a decreasing injury rate with age. In lifestyle activities (p = 0.038), in turn, there was an increasing trend of injuries with aging. In commuting activities (hazard ratio, HR 5.99, 95 % CI 1.40 to 25.6), the risk of knee injury was significantly higher in women than in men. In conclusion, the knee injury risk per exposure hours is almost ten times lower in commuting and lifestyle activities compared to recreational and competitive sports. The knee injury risk is especially high in the age group of 15 to 25 years, especially in various team sports and ball games. At population level, however, widely practiced low-to-moderate intensity activities with relatively low injury risk per exposure hours produce a large absolute number of knee injuries.