OBJECTIVE To evaluate the alterations in plasma gonadotropin and sex steroid levels in obesity. METHODS A study was conducted on 77 obese women, 30 with ovulatory menstrual cycles (group I) and 47 with chronic anovulation (group II), who were compared to 15 women of normal weight and with ovulatory cycles (group C). Obesity was determined on the basis of body mass index. Blood samples were obtained during the follicular phase of the cycle for measurement of follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), testosterone (T), androstenedione (A), dehydroepiandrosterone sulfate (DHEAS), estrone (E1) and estradiol (E2). RESULTS Elevated LH, T, A and E1 levels were detected in group II when compared to group I and group C. A levels were also elevated (P < or = .05) in group I when compared to group C. There was no significant difference between the three groups in FSH, PRL, DHEAS or E2 levels. CONCLUSIONS The elevated A levels in group I obese women show that even when ovulating, obese women are hyperandrogenic. This excessive A may activate the aromatizing system in adipose tissue, generating E1. When this occurs, the increase in E1 may trigger an increase in LH, with consequent ovarian hyperstimulation, leading to increased T and anovulatory cycles.