The aim of this study is to analyze the impact of physical inactivity from middle age to early old age on functional ability at age 75. Physical activity is measured both as cumulated activity from age 50 to 60 to 70 and at three separate points in time. Three hundred eighty-seven men and women born in 1914 and living in seven municipalities in the western part of the County of Copenhagen were followed for 25 years with examinations in 1964, 1974, 1984 and 1989. Analyses were conducted with physical inactivity as an independent variable (accumulated and separately for each point in time) and smoking, sex, school education, household composition, chronic disease at baseline and functional ability at age 70 as possible confounders. There was a strong association between physical inactivity at age 70 and disability at age 75. However, the analyses showed no effect of cumulated physical inactivity from age 50 to 60 to 70 on disability at age 75 when adjusting for functional ability at age 70. Physical inactivity is a risk factor for disability among old people. Thus, old people should be encouraged to take up and maintain physical training throughout the aging process.