Thirty-one patients with primary dysmenorrhoea were treated in a double-blind, six-period, cross-over clinical trial with tiaprofenic acid, naproxen sodium and a placebo in randomized order, each for 2 consecutive cycles. Complete disappearance of the symptoms or pronounced therapeutic effects were obtained with tiaprofenic acid, naproxen sodium and the placebo in 74%, 65% and 35% of cases, respectively, while these treatments were ineffective in 3%, 6% and 38% of cases, respectively. Tiaprofenic acid was superior to the placebo for relieving pelvic pain and overall discomfort and for reducing the need for bed-rest. Naproxen sodium compared favourably with the placebo with respect to pelvic pain and overall discomfort. The effects of tiaprofenic acid and naproxen sodium were not significantly different. Tiaprofenic acid had no side-effects, whereas tiredness was experienced in 3 cases of naproxen sodium treatment. The results indicate that tiaprofenic acid is a useful alternative for the treatment of primary dysmenorrhoea.