Nine women with acanthosis nigricans and masculinization, who did not appear to have any of the reported syndromes associated with acanthosis nigricans, were studied to characterize the clinical, biochemical, and ovarian morphologic features of their disorders. These patients had the clinical and biochemical profiles of polycystic ovarian disease. All acanthosis nigricans subjects had significant insulin resistance when insulin binding to both circulating monocytes and erythrocytes was compared to the control subjects. Microscopic examination of the ovaries revealed no evidence of recent normal ovulation, sclerosis of the ovarian cortex, follicle cysts, and stromal hyperthecosis. The authors conclude that ovarian stromal hyperthecosis and insulin resistance are consistent findings in the present type of patient. This study provides further evidence supporting a relationship between insulin resistance and human ovarian function.