Objective: To evaluate the impact of long-term purified FSH (pFSII) therapy in male partners before intracytoplasmic sperm injection ICSI). Design: Prospective, randomized, controlled study. Setting: Large university-based IVF unit. Patient(s): Seventy-eight patients made up the study and control groups (39 patients each). All patients had severe male factor infertility. Intervention(s): Induction of ovulation, oocyte retrieval, ICSI, and ET were carried out in both groups. In the study group, male partners received pFSH (75 IU FSH, <1 IU LH) for greater than or equal to 50 days before oocyte retrieval. Main Outcome Measure(s): Fertilization rate, embryo quality, implantation and pregnancy rates. Result(s): Fertilization and pregnancy rates were higher in the study group (68% and 35.9%, respectively) were higher than in the control group (59% and 17.9%, respectively), although the differences did not reach statistical significance. The implantation rate was significantly higher in the study group (15.5% versus 6.5%). The study group showed a trend toward a higher number of better-quality embryos per transfer (mean +/- SD, 2.2 +/- 1.6 versus 1.6 +/- 1.6). Conclusion(s): Purified FSH therapy in male partners before ICSI improves implantation rate. Improved embryo quality may be a contributory factor. (Fertil Steril(R) 1999;72:670-3. (C) 1999 by American Society for Reproductive Medicine.).