Assessment of the temporal relationship between the changes in serum gonadotropin and corpus luteum function in response to clomiphene citrate (Clomid) was made in 7 patients (8 cycles) with polycystic ovary syndrome (PCO). Daily serum FSH, LH and progesterone concentrations were determined before, during and after Clomid treatment until menses ensued. In most instances, Clomid induced a transient rise in LH and FSH during the treatment (Clomid peak), which was followed by a pattern of gonadotropin secretion closely resembling that observed in the normal ovulatory cycles. An average latent phase of 6 days between the Clomid peak and preovulatory surge was observed. In 2 patients, apparent ovulation was induced with the absence of “post-Clomid nadir.” The magnitude of increase was greater for LH than for FSH. “Ovulatory” response as judged by the elevation in progesterone concentration occurred in 7 of 8 cycles and one patient conceived. However, the day of maximum concentration of progesterone occurred sooner and declined earlier than those observed in the normal ovulatory cycles and thus resulted in a short luteal phase of 10 days or less in over half of the patients studied. These data suggest that in patients with PCO, despite high erratic secretion of LH, an adequate pituitary store may be present. The high frequency of inadequate luteal phase in Clomid-induced “ovulation” is probably related to the inappropriate timing between gonadotropin stimulation and follicular maturation.