Many factors influence the decision to initiate and continue to breastfeed. Our study evaluates the relation between maternal cigarette smoking and prevalence of breastfeeding. A telephone questionnaire was administered by nurses to mothers of 879 newborns from the Quebec City area approximately two weeks after delivery. Among the 666 babies whose mothers did not smoke, 62.6% were breastfed. This proportion declined to 37.5%, 17.7% and 14.6% among babies whose mothers smoked 1 to 10, 11 to 20 and more than 20 cigarettes per day, respectively. The dose-effect association between the prevalence of breastfeeding and the number of cigarettes smoked daily by the mother persisted after adjustment in a binomial regression model for maternal age, education and home district area. Pregnant women who smoke should be strongly encouraged to quit smoking and to breastfeed their babies.