We have measured the plasma concentrations of sex steroids and sex hormone‐ binding globulin (SHBG) in twenty‐three massively obese women and ten age‐matched lean female volunteers. In the obese women increased plasma testosterone (obese 3�2 � 0�5 nmol/1 controls 1�7 � 0�5 nmol/1, P < 0�3) and androstenedione concentrations (obese 9�7 � 1�2 nmol/1, controls 4�4 � 0�6 nmol/1, P= < 0�01) an increased ratio of oestrone:oestradiol (obese 2�4 � 0�4, controls 1�0 � 0�1, P= < 0�1) and decreased SHBG levels (obese 30 � 4 nmol/1, controls 60 + 8 nmol/1, P= <0�001) were found. Obesity differed from the polycystic ovary syndrome (in which a similar pattern of changes of sex steroid concentrations and binding are seen) in that it was associated with normal increases in serum luteinizing hormone (LH) follicle stimulating hormone (FSH) levels in response to the administration of LHRH. We conclude that the common occurrence of menstrual abnormalities in obesity results from abnormal secretion and binding of sex steroids. In addition, the unaltered secretion of LH and FSH in the presence of such changes is evidence for a disorder of hypothalamic function.