The relationship between contraceptive pill use, urinary sodium and blood pressure was investigated in a random sample of the premenopausal female inhabitants of 2 small Belgian towns. Systolic and diastolic pressure tended to be higher in 58 present pill users (126 +/- 11/75 +/- 9 mm Hg: mean +/- standard deviation) than in 110 women not currently taking the contraceptive pill (123 +/- 11/74 +/- 8 mm Hg). Their urinary sodium excretion averaged 149 +/- 47 and 143 +/- 57 mEq/24 h, respectively. Systolic pressure was positively associated with 24-hour urinary sodium in pill users (r = +0.41; p less than 0.01) and in those not on the pill (r = +0.19; p less than 0.05). This association was independent of age and persisted after adjusting for body weight and pulse rate in pill users, but was no longer apparent in the others when body weight, pulse rate, and age were taken into account. Thus, pill use was associated with an elevation of arterial pressure. In subjects taking the pill a positive association was observed between the urinary sodium excretion and systolic pressure. It is therefore possible that the systolic pressure of pill users becomes responsive to their usual sodium intake.