A prospective panel cohort design was used to investigate whether mental disorders common in the general population increase the likelihood of increased cigarette smoking at 12 months follow-up. By 1995, the last year for which data were available, a random sample of 12,057 persons aged 16-75 years residing in private households in Great Britain had been recruited. At each of five annual waves, the main exposure, past mental disorder, was derived from assessments of psychiatric morbidity as measured by the General Health Questionnaire-12. Increased cigarette smoking was derived from observations of number of cigarettes smoked and was defined by an increase of five or more per day relative to the previous calendar year. After logistic regression analysis, persons with a common mental disorder were about 30% more likely to have increased their cigarette smoking over the previous year (odds ratio = 1.29, 95% confidence interval: 1.16, 1.43). The estimated effect in the youngest (16-21 years) and oldest (51-75 years) age groups was higher than that in the middle (31-50 years) age group (odds ratios = 1.50, 1.57, and 1.12, respectively; test for interaction, chi2 = 6.8 (3 df), p = 0.078). These findings indirectly support the hypothesis that common mental disorders may have an enduring effect of increasing cigarette smoking a year later.