OBJECTIVE:To determine whether folliculogenesis is impaired in patients with endometriosis. DESIGN:Prospective study. SETTING:Yamagata University School of Medicine, Yamagata, Japan. PATIENT(S):Thirty women participating in an IVF program. INTERVENTION(S):The patients were divided into four groups according to the cause of their infertility: tubal factor (T), n = 7; male factor (M), n = 7; idiopathic (I), n = 7; and endometriosis (E), n = 9. Granulosa cells were obtained from the follicular fluid of each patient and analyzed by flow cytometry. MAIN OUTCOME MEASURE(S):The percentage of granulosa cells in each cell-cycle stage. RESULT(S):The mean (+/- SD) rate of apoptosis in the granulosa cells obtained from the patients with endometriosis was the highest among the four groups (T = 11.7% +/- 3.3%; M = 5.6% +/- 3.8%; I = 9.6% +/- 5.1%; and E = 18.6% +/- 9.6%). The percentage of S-phase granulosa cells was significantly higher in the patients with endometriosis than in all the other patients combined (E = 12.5% +/- 6%; T + M + I = 9.3% +/- 2.9%). The percentage of G2/M-phase granulosa cells was significantly lower in the endometriosis group than in the other three groups combined (E = 2.3% +/- 2.5%; T + M + I = 4.6% +/- 2.1%). CONCLUSION(S):Endometriosis impairs the cell cycle in granulosa cells. This phenomenon may have a detrimental effect on folliculogenesis.