The personality scores at 16 years of age of 2753 people, all members of the National Survey of Health and Development, were related, in a follow-up study, to cigarette smoking behaviour in their young adult years. Survey members who recorded high neuroticism scores were found to be more likely to smoke than those with low scores and, among the smokers, deep inhalers formed the most neurotic group. Extraverts were more likely to smoke than introverts, the mean extraversion score being greatest for the male smokers with a high daily consumption of cigarettes. The personality scores were found to have some power in predicting changes in smoking behaviour. Neurotics and extraverts who had not started to smoke by the time of completing the personality inventory at 16 were more likely than the stable and introverted to take up the habit subsequently. Among survey members who were regular smokers at the time of completing the personality inventory the proportion giving up smoking by the time they reached the age of 25 years was related to consumption level recorded at 20 years and the personality scores recorded at 16, stable extraverts among the men being most likely to stop smoking.