To investigate the changes in serum leptin concentration in the conception cycle of severe ovarian hyperstimulation syndrome (OHSS). Prospective longitudinal study of 66 IVFETcycles 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. To investigate the changes in serum leptin concentration in the conception cycle of severe ovarian hyperstimulation syndrome (OHSS). Prospective longitudinal study of 66 IVFETcycles between May 2000 and June 2002. Infertile patients undergoing at a Japanese medical school. 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 period I, on the first day of ovarian stimulation with FSH; period II, at hCG administration before oocyte retrieval; period III, 7 days after The serum leptin concentration was low in the OHSS group compared with that of the non-OH SS 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 The change pattern in serum leptin concentration might provide a great contribution to the pathophysiology development of OHSS patients during assisted reproductive treatment.