MR imaging at 1.5 T was used to investigate variations in breast parenchyma during the menstrual cycle. Seven subjects were examined twice weekly over at least one menstrual cycle. A three-point Dixon technique (TE = 19 msec, TR = 2000 msec) provided images of fat, water, and static magnetic field (Bo), from which two quantitative whole breast parameters were calculated: the mean relative volumetric water content, <WC>, and the mean volumetric fibroglandular fraction, <FV>. Four of seven subjects showed unequivocal cyclic variations in <WC> and <FV> consistent with expected histologic changes; <WC> and <FV> values were elevated during menses and reduced in mid-cycle. The maximum deviation measured for each of the four subjects was < or = 10% in <WC> and <FV> units. These variations probably do not influence significantly the clinical interpretation of unenhanced MR breast images. Quantitative measurements of breast parenchyma, however, should recognize these effects.