We evaluated the effect of smoking on the risk of cervical dysplasia in a large case-control study, obtaining smoking information from a mail questionnaire. We observed higher risks with increasing numbers of cigarettes smoked, but not with longer duration of smoking. The risk for ex-smokers was between those of current smokers and nonsmokers and was lower for women who had stopped more than 10 years ago than for those who had quit more recently. Starting to smoke at an early age did not increase the risk of cervical dysplasia.