In a prospective, comparative, dose-finding study, the minimal effective dose of recombinant human LH (rhLH) required to induce final follicular maturation and early luteinization in patients undergoing in vitro fertilization and embryo transfer was determined. In addition, the efficacy and safety of rhLH were compared with urinary human CG (u-hCG). A total of 259 infertile women, aged 18-39 yr, were enrolled in the study. After pituitary desensitization using a GnRH agonist, rhFSH was administered for ovarian stimulation. Patients then received either rhLH or u-hCG to achieve final follicular maturation. The doses of rhLH administered were 5,000, 15,000, 30,000, or 15,000 + 10,000 IU (second injection administered 3 days after the first injection; 129 patients), and those of u-hCG were consistently 5,000 IU (121 patients). Ovum pick-up was performed 34--38 h after rhLH or u-hCG injection. After fertilization in vitro, up to three embryos were replaced in the uterine cavity. The numbers of oocytes retrieved after u-hCG or rhLH administration were not significantly different between the four different doses of rhLH, when compared with each corresponding u-hCG group, nor when compared with the pool of all u-hCG groups. Similarly, there were no statistically significant differences in: the number of oocytes retrieved per follicle with a diameter of over 10 mm on the day of u-hCG or rhLH administration; the number of patients with at least one oocyte retrieved; oocyte nuclear maturity; oocyte potential for fertilization; the number of embryos; the number of total, biochemical, and clinical pregnancies; and the embryo implantation rate. However, in many of these parameters, the lowest dose of rhLH seemed suboptimal when compared with the higher dose. In terms of safety, rhLH was well tolerated at a dose of up to 30,000 IU. Moderate ovarian hyperstimulation syndrome (OHSS) was reported in 12.4% of patients who received u-hCG and 12.0% of patients who received two injections of rhLH. No moderate or severe OHSS was reported in patients who received a single dose of rhLH up to 30,000 IU. The results show that a single dose of rhLH is effective in inducing final follicular maturation and early luteinization in in vitro fertilization and embryo transfer patients and is comparable with 5,000 IU u-hCG. A single dose of rhLH results in a highly significant reduction in OHSS compared with hCG. The dose of rhLH giving the highest efficacy to safety ratio was between 15,000 and 30,000 IU.