The role of optimistic bias in beliefs about the health consequences of cigarette smoking was examined by assessing young adults' perceptions of the health risks of smoking, both for themselves and for the average smoker. A sample of 189 respondents aged 18 to 30 years was randomly drawn from the telephone directory. Respondents were interviewed over the telephone using a highly structured interview schedule. Smokers and nonsmokers gave similar estimates of risk for the average smoker of the same age and sex, but differed in their estimates of their own risk of lung cancer, number of friends against smoking, proportion of smokers who would like to quit, smoking prevalence, years until others get lung cancer, and knowledge of health risks. Optimistic bias was assessed among smokers by comparing personally relevant with population-relevant variables. Smokers evaluated themselves more positively than the average smoker of the same age and sex on number of friends against smoking, risk of lung cancer, likelihood of success in quitting, risk of emphysema, and risk of heart disease. The study extends previous research by providing evidence of optimistic biases in perceptions of people against smoking, and perceptions of success in quitting smoking. Optimistic bias may be one factor explaining why people continue to engage in negative health behaviours, despite having knowledge of the associated consequences.