The purpose of this study was to evaluate blood magnesium (Mg) measures across the menstrual cycle in women with premenstrual syndrome (PMS) and control women. Longitudinal determinations of plasma, red blood cell (RBC) and mononuclear blood cell (MBC) Mg were made in 26 women with prospectively confirmed PMS and in a control group of 19 women. Data were analyzed using analysis of variance with repeated measures and Spearman rank correlations. Significant diagnostic group effects were observed for RBC and MBC Mg concentrations (p < 0.05). These effects reflected lower Mg concentrations in PMS patients at each sampling time. No significant effects were observed for either plasma Mg or MBC Mg content, nor were there significant time by diagnosis effects for any of the measures. Consistent with earlier studies, we found decreased RBC Mg concentrations and additionally observed decreased MBC Mg concentrations in women with PMS. However, neither of these relative deficits were confined to the luteal phase.