1. Changes in blood pressure during the normal menstrual cycle are not well documented, and previous studies have given conflicting results.2. Thirty normotensive women and ten mildly hypertensive women measured their blood pressure at home each morning for 6 weeks, under standardized conditions, using a UA-751 semi-automatic sphygmomanometer. All had normal menstrual cycles and subjects entered the study at difference phases of the cycle.3. Blood pressure was higher at the onset of menstruation than at most other phases of the cycle (systolic blood pressure, P < 0.05; diastolic blood pressure, P < 0.001). Adjusted diastolic blood pressure was higher in the follicular than in the luteal phase (mean difference 1.23 mmHg, P < 0.001). Similarly, blood pressure was lower during days 17-26 than during the remainder of the cycle (adjusted mean difference in systolic blood pressure -0.65 mmHg, P = 0.07; adjusted mean difference in diastolic blood pressure - 1.19 mmHg, P < 0.001).4. Similar patterns were seen in normotensive and hypertensive subjects, and changes in plasma 17-beta-oestradiol and progesterone concentrations were also similar in the two groups.