OBJECTIVE: To assess the sensitivity and specificity of common clinical tests used for the diagnosis of luteal phase defect (LPD). DESIGN: The sensitivity and specificity of these tests for predicting low integrated P levels over the luteal phase were calculated. SETTING: Outpatient reproductive endocrinology and infertility clinic at a university medical center. PATIENTS: Fifty-eight strictly defined normal women were used to determine normal integrated luteal phase P levels. The study population was a separate 34 women who either were normal (n = 15) or were being evaluated for infertility or recurrent abortion (n = 19). These 34 study subjects all had the following tests performed in the same menstrual cycle: daily reproductive hormone levels, daily assessment of preovulatory follicle size, late luteal endometrial biopsies, and BBT charts. MAIN OUTCOME MEASURES: Basal body temperature, maximum preovulatory follicle size, dated endometrial biopsies, and serum P levels (single and multiple) were used in an attempt to predict which patients had low integrated P levels. RESULTS: Unacceptably low sensitivity and/or specificity levels were found for the following tests: appearance of BBT charts, luteal phase length, and preovulatory follicle diameter. Timed endometrial biopsy was found to have marginally acceptable sensitivity and specificity levels whether dated by next menstrual period or midcycle events. The best test for the prediction of low integrated P was a single serum P level from the midluteal phase that was < 10 ng/mL (31.8 nmol/L) or a sum of three random serum P measurements that was < 30 ng/mL (95.4 nmol/L) (also obtained in the midluteal phase). CONCLUSIONS: Luteal phase defect is a relatively uncommon but important cause of infertility and/or habitual abortion. The recommended test for the determination of LPD is a midluteal phase single serum P level < 10 ng/mL or the sum of three serum P levels that is < 30 ng/mL. The endometrial biopsy is a second line test that is only recommended when LPD needs to be evaluated in a treated cycle (ovulation induction or supplemental P).