To investigate the changes in serum leptin concentration in the conception cycle of severe ovarian hyperstimulation syndrome (OHSS). Prospective longitudinal study of 66 IVF-ET cycles between May 2000 and June 2002. Infertility outpatient clinic at a Japanese medical school. Infertile patients undergoing IVF-ET cycles at the infertility outpatient clinic were divided into three groups consisting of group 1 (conception-cycle patients with severe OHSS, n = 9), and group 2 (conception cycle, non-OHSS, n = 28), and group 3 (nonconception cycle, non-OHSS, n = 29). Blood samples were collected at five different periods. Period I, on the first day of ovarian stimulation with FSH; period II, at hCG administration before oocyte retrieval; period III, 7 days after oocyte retrieval; period IV, 14 days after oocyte retrieval; and period V, 21 days after oocyte retrieval. Serum leptin concentration. The serum leptin concentration was low in the OHSS group compared with that of the non-OHSS groups in all serum samples, with significant levels at periods III (vs. groups 2 and 3; P<.05) and IV (vs. group 3; P<.01). A twofold increase of leptin concentration from period I to period II (P<.05) was observed in all groups. The change pattern in serum leptin concentration might provide a great contribution to the pathophysiology development of OHSS patients during assisted reproductive treatment.