: ObjectiveTo determine whether recombinant follicle-stimulating hormone (rFSH) should be administered intramuscularly (IM) or subcutaneously (SC) to obese women.: DesignRandomized, single-center, two-way crossover study.: SettingAcademic clinical research center.: Subject(s)Nineteen healthy women of reproductive age with body mass indices of 19.9 kg/m2–42.8 kg/m2.: Intervention(s)Leuprolide acetate 3.75 mg IM to achieve pituitary down-regulation as determined by serum E2 levels. Subjects were then given a single dose of 300 IU rFSH either IM or SC. Multiple blood sampling was performed over the next two weeks, and after retreatment with leuprolide, a second 300 IU rFSH dose was given via the other administration route.: Main outcome measure(s)Serum samples were analyzed in duplicate for follicle-stimulating hormone (FSH) using a standard radioimmunoassay in a single run. Maximum concentrations (Cmax), times to Cmax (Tmax), and extent of absorption (area under curve [AUC]) with IM vs. SC administration were compared using paired analysis.: Result(s)Maximal concentrations were achieved within 24 hours with both SC and IM routes. No significant differences were found in Cmax, Tmax, or AUC with SC vs. IM administration. A decline of AUC occurred among subjects of higher body mass index (BMI) with rFSH given either SC or IM. Subcutaneous administration achieved AUCs comparable to IM administration in both normal-weight and obese subjects.: Conclusion(s)Our data indicate that the SC administration of rFSH is appropriate for women regardless of body mass. [Copyright &y& Elsevier]