The authors studied the effect of nitrous oxide on success rates for in vitro fertilization and pregnancy in women undergoing laparoscopy for oocyte retrieval. Ninety-eight patients in an in vitro fertilization program were randomly assigned to an anesthetic regimen including either 0.7% (end-tidal) isoflurane with 60% nitrous oxide in oxygen, or 1.4% (end-tidal) isoflurane in oxygen. Success rates for fertilization and pregnancy in 44 additional patients who declined randomization were also studied. Among the 51 randomized patients who did not receive nitrous oxide, 192 oocytes were obtained and 122 fertilized (63.5%), resulting in eight pregnancies (16.3%). From the 47 randomized patients given nitrous oxide, 168 oocytes were retrieved and 114 fertilized (67.9%), resulting in nine pregnancies (19.1%). No significant differences between rates of fertilization or pregnancy emerged between groups. Such differences would have been found with an 80% probability had nitrous oxide had a 20% effect on oocyte fertilization.