Aims/hypothesis The aim of the study was to clarify whether a therapeutic intervention focused on lifestyle modification affected the incidence of vascular complications in patients with established diabetes. Methods A total of 2,033 eligible Japanese men and women aged 40–70 years with type 2 diabetes from 59 institutes were randomised to a conventional treatment group (CON), which continued to receive the usual care, and a lifestyle intervention group (INT), which received education on lifestyle modification regarding dietary habits, physical activities and adherence to treatment by telephone counselling and at each outpatient clinic visit, in addition to the usual care. Randomisation and open-label allocation were done by a central computer system. Primary analysis regarding measurements of control status and occurrence of macro- and microvascular complications was based on 1,304 participants followed for an 8 year period. Results Although status of control of most classic cardiovascular risk factors, including body weight, glycaemia, serum lipids and BP, did not differ between groups during the study period, the incidence of stroke in the INT group (5.48/1,000 patient-years) was significantly lower than in the CON group (9.52/1,000 patient-years) by Kaplan–Meier analysis (p = 0.02 by logrank test) and by multivariate Cox analysis (HR 0.62, 95% CI 0.39–0.98, p = 0.04). The incidence of CHD, retinopathy and nephropathy did not differ significantly between groups. Lipoprotein(a) was another significant independent risk factor for stroke. Conclusions/interpretation These findings suggest that lifestyle modification had limited effects on most typical control variables, but did have a significant effect on stroke incidence in patients with established type 2 diabetes. Clinical Trial Registration: UMIN-CTR C000000222 Funding: The Ministry of Health, Labour and Welfare, Japan Electronic supplementary material The online version of this article (doi:10.1007/s00125-009-1622-2) contains a list of members of the Japan Diabetes Complications Study Group, which is available to authorized users.