Levels and changes in self-reported physical activity over a 7-year period were examined to determine tracking and to estimate the proportion of total cohort change attributable to secular trends. A population-based sample of 2,328 men and 2,787 women aged 18-30 years at baseline (52% black and 48% white) from Birmingham, Alabama, Chicago, Illinois, Minneapolis, Minnesota, and Oakland, California, were examined four times between 1985-1986 and 1992-1993. The intraclass correlation for up to four measures was 0.57 for the entire sample, varying between 0.57 for white men and 0.42 for black women, indicating a moderate tendency for tracking. The energy expenditure in physical activity at each examination was greatest in black men and, compared with black men, about 5% less in white men, 30% less in white women, and 50% less in black women. The total cohort decrease in mean physical activity was approximately 30% in each race-sex group. The secular trend accounted for 38% of the total cohort change in black men, 43% in black women, 52% in white men, and 81% in white women. Physical activity declined sharply during the early years of adulthood, partly because of secular trend. Young adults are therefore an important target group for physical activity promotion programs to reverse individual and populationwide declines prior to middle age.