Serum samples were collected from 27 women with incapacitating primary dysmenorrhoea and from 16 normal women in the early part of the cycle (between day 3 to 6), at midcycle (between days 13 to 16) and in the late part of the cycle (between days 23 to 26) for determination of FSH, LH, prolactin (PRL), 17 beta-oestradiol (E2) and progesterone (P). The gonadotrophin and P levels showed normal and similar ovulatory patterns in both groups. The PRL concentrations (mean +/- SD) in dysmenorrhoeic women were lower than normal in the early part (9.2 +/- 4.0 ng/ml vs 14.5 +/- 7.3 ng/ml, p less than 0.01) and in the late part of the cycle (11.7 +/- 5.2 ng/ml vs 16.5 +/- 10.2 ng/ml, p less than 0.05), but not so at midcycle (9.1 +/- 2.8 ng/ml vs 10.4 +/- 4.4 ng/ml, p greater than 0.05). The E2 level was higher than normal in dysmenorrhoeic women in the late cycle (163.0 +/- 76.7 pg/ml vs 93.3 +/- 64.3 PG/ML, p less than 0.01), but apart from this the ratio of E2/P did not differ between the groups. These hormonal changes may be related to an excessive production of endometrial prostaglandins in primary dysmenorrhoea.