Menstrual history in relation to nutritional status and lung function was assessed in 45 adolescent and adult patients with cystic fibrosis (CF). Twenty patients had regular menstrual cycles (of whom six had been pregnant), 12 had primary or secondary amenorrhoea, 11 had irregular cycles and two were premenarcheal. Patients with regular menstruation had a significantly higher body mass index and percentage body fat (estimated from skinfold thickness measurements) than those with amenorrhoea (P less than 0.001), and also better lung function (P less than 0.01). A linear discriminant analysis indicated that of the four indices studied percentage body fat was the most important predictor of menstrual function. Ovarian and uterine morphology were studied by ultrasonography in 28 patients and 17 healthy controls. In 10 patients multicystic ovaries were noted. Even in patients with regular cycles uterine size was reduced (P less than 0.01) and in those with amenorrhoea appeared to be related to thinness. Amenorrhoea in CF is an appropriate adaptive response in preventing pregnancy, but the resultant oestrogen deficiency and psychological effects must be considered.