Liability to neural tube defects is increased by maternal dietary deficiency, and children with neural tube defects show a possibly related seasonal variation in date of birth. Since maternal dietary insufficiency may also increase liability to cleft lip, with or without cleft palate (CLP), we wondered if children with CLP would also show a seasonal variation in birth date. The multifactorial-threshold model predicts that any such effect would be more apparent in males than females, since CLP is more frequent in males. Month of birth was obtained from records of 598 children with CLP seen at The Montreal Children's Hospital between 1950 and 1996. Children with syndromes or associated malformations were excluded. There was a significant tendency for children with CLP to be born more often in the summer than in winter. The difference was greater in males than in females. The seasonal fluctuation in month of birth of children with CLP is consistent with the presence of an environmental factor increasing liability, with a maximal effect in November-December. This might be related, at least in part, to a seasonal fluctuation in maternal nutrition. The data support the prediction that analyzing the data for the sexes separately would amplify the effects of variation in liability for a multifactorial threshold trait that has a different frequency in males and females. This approach could be useful in the study of other gene-environment interactions.