BACKGROUND: Intensive lifestyle interventions in well-controlled settings are effective in lowering the risk of chronic diseases such as type 2 diabetes (T2DM) and cardiovascular diseases (CVD), but there are still no effective lifestyle interventions for everyday practice. In the Hoorn Prevention Study we aimed to assess the effectiveness of a primary care based lifestyle intervention to reduce the estimated risk of developing T2DM and for CVD mortality, and to motivate changes in lifestyle behaviors. METHODS: The Hoorn Prevention Study is a parallel group randomized controlled trial, implemented in the region of West-Friesland, the Netherlands. 622 adults with ≥10% estimated risk of T2DM and/or CVD mortality were randomly assigned and monitored over a period of 12 months. The intervention group (n=314) received a theory-based lifestyle intervention based on an innovative combination of motivational interviewing and problem solving treatment, provided by trained practice nurses in 12 general practices. The control group (n=308) received existing health brochures. Primary outcomes was the estimated diabetes risk according to the formula of the Atherosclerosis Risk In Communities (ARIC) Study, and the estimated risk for CVD mortality according to the Systematic COronary Risk Evaluation (SCORE) formula. Secondary outcomes included lifestyle behavior (diet, physical activity and smoking). The research assistants, the principal investigator and the general practitioners were blinded to group assignment. Linear and logistic regression analysis was applied to examine the between-group differences in each outcome measure, adjusted for baseline values. RESULTS: 536 (86.2%) of the 622 participants (age 43.5 years) completed the 6-month follow-up, and 502 (81.2%) completed the 12-month follow-up. The mean baseline T2DM risk was 18.9% (SD 8.2) and the mean CVD mortality risk was 3.8% (SD 3.0). The intervention group participated in a median of 2 sessions. Intention-to-treat analyses showed no significant differences in ...