Naproxen tablets and suppositories were compared, in the treatment of primary dysmenorrhea, in a double-blind cross-over trial. The results on 32 patients treated during 128 menstruations with either tablets and suppositories were analysed. Both naproxen tablets and suppositories produced a significant but similar overall relief of dysmenorrhea, although the tablets had a better effect in relieving spasmodic pain than the suppositories (p < 0.05). Occasions of failure to obtain relief were not related to the occurrence of vomiting or diarrhea during the trial. Vomiting seems not to be responsible for the therapeutic failures of oral treatments with prostaglandin-synthetase inhibitors in primary dysmenorrhea.