Results of antihypertensive treatment were analyzed in a group of 5,209 hypertensive patients referred in two hypertension Clinics in Paris from 1976 to 1985. Patients were included in the study if they fulfilled the following criteria: 1) at least four visits in the clinic, 2) follow-up period greater than 6 months, 3) initial diastolic blood pressure greater than or equal to 90 mmHg and/or presence of an antihypertensive treatment. After a mean follow-up period of 43 months, blood pressure was reduced from 177/105 mmHg to 148/89 mmHg. However, in spite of at least two antihypertensive drugs, 16.7 p. 100 of these patients had a diastolic blood pressure above 95 mmHg at the end of the follow-up period and were defined as uncontrolled hypertensives (UH). By comparison with controlled hypertensives (CH), UH patients were more frequently males (67.9 vs 56.6 p. 100, p less than 0.001), had a greater known duration of hypertension (11.6 vs 8.7 years, p less than 0.001), and presented at the first visit with higher blood pressures (188/113 vs 174/103 mmHg, p less than 0.001/p less than 0.001), despite a higher rate of antihypertensive treatment (66.6 vs 53.8 p. cent, p less than 0.001), a higher ponderal index (26.3 vs 25.3/kg/m2, p less than 0.01) and a higher prevalence of end-organ damage (23.2 vs 16.3 p. 100, p less than 0.001). Sokolov index, serum creatinine and uric acid levels were higher among UH than among CH patients.(ABSTRACT TRUNCATED AT 250 WORDS)