In a prospective study of normal couples discontinuing contraception to begin a pregnancy, the days of ovulation were estimated by hormonal assay of daily urine specimens. No hormonal interventions were used. Length of the follicular phase (from onset of menses to ovulation) was found to be related to the sex of the baby among 133 pregnancies that survived to delivery. Conception cycles with short follicular phases (early ovulation) tended to produce boys, while those with long follicular phases tended to produce girls. This relationship is consistent with other data and could explain sex-related differences in the length of gestation and the observed excess of same-sex pairs among dizygotic twins.