Objective. To evaluate an intervention programme on smoking cessation in patients with diabetes mellitus in primary healthcare. Design. Regional controlled intervention study. Setting. Seventeen primary healthcare centres in Sweden. Intervention. In the intervention centres, nurses with education in diabetes were given one half-day of training in motivational interviewing and smoking cessation. An invitation to participate in a smoking cessation group was mailed to patients from the intervention centres followed by a telephone call from the patient's diabetes nurse. The nurses who intervened were specially educated in smoking cessation. The control group received a letter containing advice to stop smoking and information about a one-year follow-up. Patients. Daily smokers with diabetes mellitus, 30–75 years of age. In the intervention centres 241 patients fulfilled the criteria and in the control centres 171 patients. Main outcome measures. Self-reported smoking habits after one year. Results. In total, 21% of the smokers accepted group treatment. After 12 months, 20% (42/211) in the intervention centres reported that they had stopped smoking and 7% (10/140) in the control centres; 40% (19/47) of the smokers who had participated in group treatment reported that they had stopped smoking. Conclusion. A computerized record system for patients in primary healthcare was used to identify diabetic patients who were smokers. The selected group was invited to a stop smoking programme. At a one-year follow-up significantly more patients in the intervention centres had stopped smoking compared with patients in the control centres.