When luteal phase relaxin concentrations were summed to give an integrated measure (pg/mL per cycle), relaxin was found to be significantly lower in those cycles with an out-of-phase endometrial biopsy. In addition, peak relaxin concentrations were lower in out-of-phase cycles compared with normal cycles. These data indicate that relaxin secretion may be related to normal luteal function and suggest that shortening of the luteal phase results in reduced relaxin production. Measurement of circulating relaxin may prove to be useful in making the diagnosis of out-of-phase biopsy and needs to be assessed for its usefulness in diagnosing abnormal luteal function.