Inositol, a simple isomer of glucose, which serves as a precursor in the phosphatidyl-inositol (PI) second messenger cycle, was shown to be effective in double-blind, placebo-controlled studies of depression, panic and obsessive compulsive disorders as well as in bulimia. The following study was designed to investigate whether inositol has beneficial effects in another disorder shown to be responsive to SSRIs: premenstrual dysphoric disorder (PMDD). Eleven female patients with PMDD diagnosed according to DSM-IV participated in a cross-over, double-blind, placebo-controlled trial. The active drug was myo-inositol, 12 g daily, whereas placebo was d-glucose administered at the same dose. Each drug was given during the luteal phase only (14 days prior to menses). For each patient treatment alternated between these two drugs for six menstrual cycles. No beneficial effect was demonstrated for inositol over placebo.