To investigate the relationship between maternal cigarette smoking and the risk of oral clefts in offspring, we examined data from the Atlanta Birth Defects Case-Control Study, which included 238 cases of cleft lip with or without cleft palate and 107 cases of cleft palate ascertained by the Metropolitan Atlanta Congenital Defects Program from 1968 through 1980. In all, 2809 infants who served as controls were frequency matched to cases by race, period of birth, and hospital of birth. Maternal periconceptional exposures to smoking were investigated through use of a structured questionnaire. Smoking exposure was defined as reported maternal smoking during the periconceptional period (from 3 months before conception to 3 months after pregnancy began). Offspring of smoking mothers were 1.6 and 2.0 times more likely than offspring of nonsmoking mothers to have isolated cleft lip with or without cleft palate and cleft palate, respectively. On the other hand, offspring of smoking mothers were not at increased risk of having cleft palate or cleft lip with or without cleft palate that are associated with other defects. Adjustment for potential confounding variables did not alter these results. A relatively modest effect of smoking may be explained by the presence of underlying etiologic heterogeneity in oral clefts and differential susceptibility to smoking. Because of the inconsistencies in the literature on the relationship between smoking and oral clefts, these results suggest the need to refine oral clefts into more homogeneous subgroups in epidemiologic studies of these defects.