Purpose. Premature ovarian failure occurs in 40%-70% of patients who receive conventional chemotherapy alone. However, the incidence is higher, 70%-100%, in patients who undergo myeloablative chemotherapy with hematopoietic stem cell transplantation (HSCT). Gonadotropinreleasing hormone (GnRH) analogs, such as leuprolide, in a continuous-release formulation, may protect the ovaries from the gonadotoxic effects of chemotherapy. In non-HSCT settings, GnRH analogs have reduced the risk for premature ovarian failure to <10%. We conducted a phase II clinical trial based on the hypothesis that giving leuprolide before conditioning chemotherapy in HSCT patients reduces premature ovarian failure incidence.Patients and Methods. Eligible patients were women aged <40 years who were HSCT candidates, were premenopausal, and had both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels <20 IU/L. Two 22.5-mg leuprolide doses were delivered in 3-month depot i.m. injections, the first within 2 months before HSCT. Patients were monitored for menstruation return, and ovarian function tests (FSH, LH, and estradiol) were done every 2 months starting 90 days after the last leuprolide dose.Results. Sixty eligible patients were enrolled, 59 underwent HSCT, and 44 were evaluable (median age, 25 years; median follow-up, 355 days). Only seven of 44 patients (16%) regained ovarian function. Of the 33 who received myeloablative regimens, six (18%) regained ovarian function. However, among the 11 who received nonmyeloablative regimens, only one (9%) regained ovarian function (p ؍ .66).Conclusion. Leuprolide did not preserve ovarian function in patients who underwent HSCT using either myeloablative or nonmyeloablative regimens. Other measures that protect ovarian function need to be investigated. The Oncologist 2012;17:233-238