The study was conducted to investigate the effect of conservative surgery of ovarian endometriomas before an ICSI cycle. Ninety-nine patients with endometrionias who were referred to an intracytoplasmic sperm injection (ICSI) cycle were enrolled in the study. The patients were prospectively randomized into two groups; group I (49 patients) underwent conservative ovarian surgery before the ICSI cycle and group II (50 patients) underwent the ICSI cycle directly. The stimulation was started 3 months after the operation in group I and directly in group II. In the ovarian surgery group, stimulation was significantly longer (14.0 days in group I and 10.8 days in group II; P = 0.001), total recombinant FSH dose was significantly higher (4575 IU in group I and 3675 IU in group II; P = 0.001), and mean number of mature oocytes was significantly lower (7.8 in group I and 8.6 in group II; P = 0.032). There was no difference in terms of fertilization (86% in group I and 88% in group II), implantation (16.5% in group I and 18.5% in group II) and pregnancy rates (34% in group I and 38% in group II). Ovarian surgery resulted in longer stimulation, higher FSH requirement and lower oocyte number, but fertilization, pregnancy and implantation rates did not differ between the groups.