['There are scarce studies of the long-term associations between leisure-time physical activity (LTPA) in midlife and phenotypic frailty in old age.', 'We studied healthy Caucasian men of high socioeconomic status (N = 514), who had participated in health checkups during the 1960s (the Helsinki Businessmen Study, Finland). In 1974, they were examined with questionnaires and clinical examinations, and LTPA was collapsed into three categories: low (n = 87), moderate (n = 256), and high (n = 171). In 2000, at mean age of 74, survivors were assessed for physical activity and frailty phenotype using the modified Fried criteria validated in our cohort. Four criteria were used: (a) weight loss > 5% from midlife or current body mass index < 21kg/m(2), (b) physical inactivity, (c) low vitality, and (d) physical weakness. Responders with 3-4, 1-2, and zero criteria were classified as frail, prefrail, and nonfrail, respectively.', 'The prevalence of frailty was 16.1%, 10.2%, and 4.7% in the low, moderate, and high LTPA groups, respectively. Higher midlife LTPA was significantly related to lower prevalence of both frailty and prefrailty in old age. After adjusting for baseline age, smoking, body mass index, blood pressure, and alcohol, the risk of frailty was 80% lower in the high LTPA group compared with the low LTPA group (odds ratio = 0.20; 95% confidence interval 0.07-0.55). This finding was supported by the relationships between the change of physical activity and frailty in old age.'] Is leisure-time physical activity in midlife related to old age frailty?