The aim of the study was to determine the prevalence of resistant hypertension in our unit and the frequency of its various causes. Of the 890 patients seen for the first time between January 1, 1993, and June 30, 1994, 257 were referred for hypertension. Of the 62 (24.1%) patients referred for "resistant hypertension", only 25 met our criteria for resistance: blood pressure > or = 160 and/or 95 mmHg on the first visit to us without "white-coat" effect and at least two additive antihypertensive drugs at optimal doses. Of the 195 patients referred for other reasons, 10 met our criteria for resistance. Thus, the prevalence of resistant hypertension is 13.5%. A cause of resistant hypertension was found in 50% of the patients referred for resistant hypertension, in 72.2% of the patients who met our criteria for resistance and in 84% of the patients referred for resistant hypertension and who met our criteria for resistance.