Background: Evidence on the association between social support and leisure time physical activity (LTPA) is scarce and mostly based on cross-sectional data with different types of social support collapsed into a single index. The aim of this study was to investigate whether social support from the closest person was associated with LTPA. METHODS: Prospective cohort study of 5395 adults (mean age 55.7 years, 3864 men) participating in the British Whitehall II study. Confiding/emotional support and practical support were assessed at baseline in 1997-99 using the Close Persons Questionnaire. LTPA was assessed at baseline and follow-up in (2002-04). Baseline covariates included socio-demographics, self-rated health, long-standing illnesses, physical functioning and common mental disorders. RESULTS: Among participants who reported recommended levels of LTPA at baseline, those who experienced high confiding/emotional support were more likely to report recommended levels of LTPA at follow-up [odds ratio (OR): 1.39, 95% confidence interval (CI): 1.12-1.70 in a model adjusted for baseline covariates]. Among those participants who did not meet the recommended target of LTPA at baseline, high confiding/emotional support was not associated with improvement in activity levels. High practical support was associated with both maintaining (OR: 1.34, 95% CI: 1.10-1.63) and improving (OR: 1.25, 95% CI: 1.02-1.53) LTPA levels. CONCLUSION: These findings suggest that emotional and practical support from the closest person may help the individual to maintain the recommended level of LTPA. Practical support also predicted a change towards a more active lifestyle.