Aim: To examine the effect of a randomized multiple risk factor intervention study for prevention of ischaemic heart disease (IHD) on the development in physical activity over a 36-month period. Methods: Two random samples (high intensity intervention, group A, n=11,708; low intensity intervention, group B, n=1,308) were invited for a health examination, an assessment of absolute risk of developing IHD, and an individualized lifestyle intervention. The participation rate was 52.5%. High-risk persons in group A were also offered diet/physical activity and/or smoking cessation group counselling. High-risk persons in group B were referred to their GP. High-risk persons were re-counselled after 12 and 36 months. The control group (group C, n=5,264, response rate=61.3%) answered a mailed questionnaire. Data were analysed using longitudinal linear regression models with random effects. Main outcome: Change in physical activity from baseline to 12-and 36-month follow-up. Results: In men, the high-intensity (group A) intervention had a beneficial effect at 12-month follow-up, whereas after 36 months both the high-intensity and the low-intensity (group B) intervention had a beneficial effect on the development in physical activity when compared with group C. This was regardless of baseline physical activity level. At 36-month follow-up there was no significant difference between groups A and B. There was no intervention effect among women. Conclusions: Only men seemed to benefit from the intervention.