50 patients suffering from primary dysmenorrhoea were treated with Magnesium (Mg 5-longoral, Artesan GmbH) in a double-blind study. After a six-month period 21 out of 25 women showed a decline of symptoms, only 4 ones reported no therapeutical effect. For monitoring treatment results prostaglandin F2 alpha (PGF2 alpha) was measured every second month. On Mg-therapy conditions we achieved a reduction of PGF2 alpha in menstrual blood to 45% of value before treatment started. As against that 90% of basic concentration were estimated from women who received a placebo. Probably, the specific therapeutical effect of Mg based on inhibition of biosynthesis of PGF2 alpha but also on its direct muscle relaxant and vasodilatory effect. Beside the PG-synthesis and ovulation inhibitors the use of Magnesium is a potential, natural opportunity to treat primary dysmenorrhoea, which is widely free of side effects.