We evaluated the potential risk factor by examining levels of habitual physical activity during middle age, and knee osteoarthritis (OA) in later years in the Framingham Cohort. Weight-bearing knee radiographs were obtained during the 18th biennial examination (1983-85), when the 1,415 subjects had a mean age of 73 years. Cohort members had been asked at 1954-57 and 1971-73 about the number of hours spent daily at various levels of physical activity, ranging from sedentary to heavy activity. Physical capacity measures, such as forced expiratory volume in one second and resting pulse rate, were also examined for association with OA. In the cohort, 97/589 men and 154/826 women had OA. We found no association between habitual physical activity and knee OA after adjusting for age, body mass index, knee injury, smoking and education. In the highest quartile of habitual physical activity compared to the least active, the aOR for men was 1.34 (95% CI: 0.66, 2.74) and for women was 1.09 (95% CI: 0.63, 1.90). There was no increase in the risk of knee OA with increasing physical activity in either men or women. Physical capacity measures were also unassociated with OA. Of interest, men with high levels of habitual physical activity had significantly elevated rates of asymptomatic osteophytes, [aOR = 2.14, (95% CI: 1.01, 4.54)], suggesting that habitual physical activity may influence development of osteophytes but not of more severe or symptomatic knee OA. In sum, habitual physical activity does not increase the risk of knee OA for men or women.