The concentrations of prostaglandin E2 and F2alpha were measured by radioimmunoassay in samples of endometrial tissue from 155 women. The results showed that the concentration of prostaglandins E2 (ng/100 mg of wet tissue; mean +/- SD) in tissue from apparently healthy women with regular menstrual cycles was 79-3 +/- 9-9 during the menstrual period. The level in the proliferative and early secretory phases fell to 11-8 +/- 4-0 and 13-3 +/- 8-3 respectively. During the late secretory phase there was a significant increase to 20-1 +/- 10-8. The corresponding values from patients with fibromyomata and without menorrhagia were similar. However, the concentration in tissue from patients with fibromyomata and menorrhagia was significantly higher (63-9 +/- 17-9) during the secretory phase of the uterine cycle. In addition, the amount was significantly higher in the groups with dysmenorrhoea (19-0 +/- 4-5 proliferative; 68-0 +/-18-15 secretory) and irregular uterine bleeding (78-3 +/- 59-0 proliferative; 87-4 +/- 64-9 secretory). The mean concentration was higher in cases of endometriosis, and significantly elevated in endometrial carcinoma. The results for prostaglandin F2alpha showed a similar pattern, but the changes were less marked in patients with irregular dysfunctional bleeding. The findings are discussed.