Objective(s): To compare granulosa cell apoptosis in patients with unexplained infertility and tubal factor. Accelerated granulosa cell apoptosis may be the cause of unexplained infertility. Study design: Setting was lVF-ET Unit of Cerrahpasa Medical Faculty, Istanbul University. GnRH analogs and gonadotropins were used for ovulation induction in patients with unexplained infertility (n = 15) and tubal factor (n = 15) undergoing in vitro fertilization and embryo transfer (IVF-ET) procedures. Following HCG injection and follicular aspiration, apoptosis of granulosa cells was assessed using the in situ DNA nick end labelling method and apoptosis rate was further determined by flow cytometry. Apoptosis rates were compared between two groups. Mann-Whitney's U-test and Student's t-test were used for statistics. Results: Apoptosis rate was significantly higher in the unexplained infertility group (33.20 +/- 35.62% versus 10.10 +/- 17.23%). Conclusion(s): Granulosa cell apoptosis seems to have a role in the etiology of unexplained infertility. (C) 2003 Elsevier Ireland Ltd. All rights reserved.