To evaluate Magnesium (Mg) effectiveness in the treatment of primary dysmenorrhea, 30 volunteer dysmenorrheic women of mean age 22.6 years were selected from the out-patients of the Dept. of Obstetrics and Gynaecology of the University of Parma during the period January-December 1989. Patients affected by secondary dysmenorrhea were excluded from the trial. The women considered were asked to self-evaluate their menstrual pain for 6 subsequent cycles using the VAS (Visual Analogue Scale). In the first cycle, as control, no drug was administered; in the following ones, every woman was given 4.5 mg oral Mg Pidolate in 3 administrations daily, from the 7th day preceding the onset of menses till the 3rd day of menstruation. Data were statistically analyzed. In Mg-treated cycles compared with the control one, first day dysmenorrhea progressively decreased, with a significant drop (p < 0.05) from the 1st to the 6th cycle. A similar trend, but not statistically significant, was seen for the 2nd and 3rd day of cycle. No side effect was remarked. These data suggest Mg administration to be a reliable therapy of primary dysmenorrhea.