Fluid intake was measured at 3 phases of the menstrual cycle in twenty women with prospectively validated premenstrual syndrome and 8 women with prospectively validated absence of cyclic symptoms. There was no cycle related change in fluid intake in either group but fluid intake was significantly lower in women with PMS than in controls at all phases of the menstrual cycle. All subjects but one denied voluntary fluid restriction. The mechanism, therefore, appears subconscious and may be related to perceived increase in fluid retention.