One hundred and forty-nine patients presenting with infertility underwent two hundred and ten endometrial biopsies as part of a routine infertility evaluation. The initial biopsy was out of phase in 44 (29.5%), confirmed by subsequent biopsy to yield a total of 28 (19%) patients with luteal phase inadequacy. Of 44 patients taking clomiphene citrate, 13 (29.5%) had out-of-phase biopsies. Although hyperprolactinemia, recurrent miscarriages, extremes of reproductive life, and clomiphene citrate administration have been associated with an increased incidence of the defect, in this series a predisposing cause could not be detected in approximately one-half of the patients. The routinely obtained endometrial biopsy provides a safe, reproducible, and adequate means of providing histologic evidence for normal endometrial development for subsequent implantation.