The plasma concentrations of sex hormones were measured in 45 oligomenorrheic and 28 regularly menstruating adolescent girls. Testosterone, androstenedione, dehydroepiandrosterone, dehydroepiandrosterone-sulfate, estradiol, progesterone, luteinizing hormone, follicle-stimulating hormone, prolactin, and sex hormone-binding globulin levels were determined in blood samples collected during one menstrual cycle. The oligomenorrheic girls had significantly higher concentrations of luteinizing hormone, androstenedione, dehydroepiandrosterone-sulfate, and free testosterone than regularly menstruating girls. Sex hormone-binding globulin concentrations were significantly lower in the oligomenorrheic group. The oligomenorrheic girls were not obese. Signs of acne or hirsutism were absent or mild. Sixty-six percent of the oligomenorrheic cycles were ovulatory. The significance of the hormonal findings is discussed.