Central obesity is an independent risk factor of mortality. Change in waist circumference after smoking cessation has not been previously reported in a population-based prospective study of both sexes. Population-based study, Inter99, 1999-2001 in Copenhagen, Denmark. 2408 daily smokers completed questionnaires and had their waist circumference and weight measured. Of these, 221 biochemically verified non-smokers and 1122 continuous smokers attended 1-year follow-up and had their waist circumference and weight measured. The mean increase in waist circumference was 3.88 cm (+/-5.4 cm) and 42% of the quitters had increased their waist circumference by > or = 5 cm. Quitters with high baseline tobacco consumption (OR 1.05, 95% CI=1.0-1.1) and quitters with self-reported reduced physical activity (OR 3.4, 95% CI=1.5-7.7) were more likely to have substantially increased waist circumference. The mean weight gain in quitters was 4.22 kg (+/-4.3 kg) and 41% had gained at least 5 kg. Female quitters gained more weight and had a higher increase in waist circumference than men. Abstinence from smoking was the most important predictor of substantial weight gain and substantial increase in waist circumference. Smoking cessation resulted in substantial increase in weight and central fat, which might attenuate some of the beneficial effects of smoking cessation. Quitters who reduced their physical activity and persons with high baseline tobacco consumption were more likely to have had a substantial increase in abdominal fat. Abstinence from smoking was the most important predictor of short-term weight gain and increase in waist circumference. It is a challenge for future smoking cessation interventions to achieve a combination of high quit rates and weight-control.