Twenty-two patients with severe or accelerated hypertension refractory to conventional hypotensive therapy have been treated with minoxidil for an extended period. Patients were divided in three groups according to different degrees of renal function or the presence of accelerated hypertension. In the first group (8 patients with normal or slightly decreased renal function) BP fell from 197 +/- 11/118 +/- 3 before minoxidil therapy to 157 +/- 7/98 +/- 2 after six months (p less than 0.001), and remained steady during the following eighteen months. In the second group (9 patients with creatinine clearance of 30 +/- 3 ml/min.1.73 m2) BP decreased from 192 +/- 9/119 +/- 4 to 147 +/- 6/91 +/- 4 at six months (p less than 0.001); renal function did not show any significant modification during the eighteen months of the study. In the third group (5 patients with accelerated hypertension) BP fell from 243 +/- 14/137 +/- 6 to 166 +/- 13/99 +/- 7 at six months (p less than 0.01). Seven patients, four in the first and three in the second group, were followed for more than six years; these patients, with mild renal insufficiency (creatinine clearance 50 +/- 4 ml/min) before minoxidil therapy, were on a protein unrestricted diet for the entire length of the study. In this group of patients BP fell from 182 +/- 9/115 +/- 3 to 150 +/- 6/96 +/- 2 after one year (p less than 0.01) and remained well controlled for the following six years or more. Renal function did not show any significant worsening over the years (monthly decrement in creatinine clearance 0.08 ml/min).(ABSTRACT TRUNCATED AT 250 WORDS)