Every year, millions of Moslems throughout the world fast from sunrise till sunset daily during the month of Ramadan, that is, experience repeated cycles of fasting-refeeding. Studies in animal models have shown that repeated cycles of fasting-refeeding may cause or exacerbate hypertension. Changes in sleeping patterns as well as changes in medication timing may also influence ambulatory blood pressure. We undertook this study in order to examine the effect of the Ramadan fast on treated hypertensive subjects. Seventeen hypertensive subjects were examined, and 24-h blood pressure monitoring was carried out twice, before and during the last week of the Ramadan. All continued their medications, which were all once-daily preparations. Twenty-four hour mean blood pressure as well as average awake and average asleep blood pressure were compared. There was no difference between mean blood pressure before and during the Ramadan (138.5 +/- 18.5/77.2 +/- 8.1 mm Hg vs 136.4 +/- 20.4/75.7 +/- 5.9 mm Hg, P-nonsignificant). Blood pressure load also did not differ before and during Ramadan (systolic load 49% vs. 44%, diastolic load 21% vs. 18%, P-nonsignificant). Weight was reduced by 1.4 +/- 1.6 kg (P < 0.002). We conclude, that according to our findings, treated, hypertensive patients may be assured that, with continuation of previous medications, traditional fasting during the month of Ramadan can be safely undertaken.