The authors investigated the associations of the amount, frequency and intensity, and type of leisure-time physical activity (LTPA) with the risk of self-reported difficulty in walking (WD) and stair climbing (SCD) over 16 years in a population-based cohort age 40–64 years at the onset of the study. Their results indicated that the risk for SCD was highest among men and women with a low amount of weekly LTPA. The risk was high also among women with weekly light LTPA compared with women with weekly vigorous LTPA. The risk for WD was highest among men who engaged in fitness activity once a week compared with men who engaged in fitness activity at least three times a week. A low amount of weekly LTPA, light LTPA twice or more a week, and LTPA for keeping fit and healthy less than three times a week are associated with future risk of mobility difficulties among middle-aged and older adults.