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This was a cohort study. A longstanding prospective study was carried out of a birth cohort born in Dunedin (New Zealand) in 1972–1973. Periodontal examinations of the study participants were carried out when they reached the age of 26 and 32 years by calibrated examiners; smoking exposure was measured by questioning at ages 15, 18, 21, 26 and 32 years. Socioeconomic status (SES) was measured at the age of 26 years by categorising adult occupation using standard New Zealand occupation indices. Dental visiting behaviour was assessed by questioning at age 25 and 32 years. Chi-square tests were used to examine the statistical significance of differences observed with categorical dependent variables. Analysis of variance was used for continuous variables. Logistic regression modelling was used to examine smoking exposure and periodontitis prevalence and incidence while controlling for sex, SES, dental plaque accumulation and the use of dental services. Complete data were available for 810 individuals of whom 48.9% had smoked at some point (31.5% were current smokers). Table 1 shows the prevalence, for a selection of smoking exposure groups of attachment loss at age 32 and the odds ratio compared with never-smokers and Table 2 the incidence of 3+ mm attachment loss. Sites with >5 mm attachment loss and were more likely to be incident cases after age 26 (OR, 5.2 and 3.2, respectively). Two thirds of new cases in individuals older than 26 years were attributable to smoking. There were no significant differences in periodontal health between never-smokers and those who had quit smoking after age 26. Current and long-term smoking in young adults is detrimental to periodontal health, but smoking cessation may be associated with a relatively rapid improvement in the periodontium.