Nineteen patients with endometriosis were selected for investigation of luteal function as determined by serum progesterone (P) concentrations at different stages of the luteal phase, and by observation of basal body temperature (BBT) profiles and endometrial histological appearances. Sixteen cases were accompanied with infertility. All data were synchronized to the day of midcycle serum luteinizing hormone (LH) peak (D0) and analyzed accordingly. In the endometriosis group, mean serum P concentrations of early, mid luteal phases and the integrated P concentration of the whole luteal phase were significantly lower than in the control group. Five cases (26.3%) had low peak P values during mid-luteal phase and/or low integrated P levels. Delayed endometrial maturation was revealed in 10 of 16 cases (62.5%). Short luteal phase, as defined by BBT, was observed in 3 of 14 cases (21.4%). A total of 12 cases were screened for all three parameters, among them, ten (83.3%) had at least one abnormality and seven (58.3%) showed two aberrant results. In summary, the incidence of luteal phase defect was higher in the endometriosis group than in controls. The pathogenic mechanism of luteal phase defect and its role in infertility are discussed.