Objectives: To investigate how trajectories of social activities, suchas spending time with family and friends, observed during a34-year period (from middle age to old age) were associated withtrajectories of mobility problems during the same time periodamong men and women.Methods: Nationally representative data from the Swedish Levelof Living Survey (LNU) and the Swedish Panel Study of the OldestOld (SWEOLD) were used. LNU data from 1968, 1981, 1991 and2000 were merged with SWEOLD data from 1992 and 2002 tocreate a longitudinal dataset with four observation periodscovering the period 1968-2002. The sample consisted of thoseaged 40-60 years at baseline who survived through the period,and participated in at least three observation periods (n=698).Trajectories of social activity were identified through clusteranalysis, and then used as predictors of mobility trajectories inmultilevel regression models.Results: Most people had a socially active life as they moved frommiddle age into old age. Five trajectories of social activity wereidentified: continuously very active, continuously active,increasing social activity, decreasing social activity, and continuouslyinactive. Upholding a very active social life was morecommon among women than men.Mobility problems increased significantly over time for bothwomen and men. Among men, decreasing activity levels overtime were associated with a faster increase in mobility problems.Among women, those who were continuously inactive or whodecreased their activity levels had higher levels of mobilityproblems, but the increase in mobility problems with age wassimilar across trajectories of social activity.Conclusions: Most men and women had high levels of socialactivity in midlife, and continued their high activity levels into latelife. Decreasing social activity was related to worse mobility inboth men and women. The nature and direction of theseassociations need to be explored further.