Menstrual fluid was collected in a contraceptive diaphragm from 16 women with primary dysmenorrhoea and 12 matched control subjects without dysmenorrhoea. Prostaglandins F2 alpha (PGF2 alpha), E2 (PGE2) and 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha) were extracted and measured using gas-chromatography: mass spectrometry (GC:MS). The concentrations of both PGF2 alpha and PGE2 were higher on days 1 and 2 in the dysmenorrhoea group than in the control group and the concentration of PGF2 alpha was higher on day 1 than on day 2 in the dysmenorrhoea group. The concentrations of 6-oxo-PGF1 alpha (the stable metabolite of PGI2) were low in both groups. These results confirm suggestions that PGF2 alpha is important in the aetiology of dysmenorrhoea and also indicate that PGE2 may be involved.