In this study, the investigation of hydralazine acetylator phenotype was undertaken for the first time in African hypertensives at Kenyatta National Hospital. A total of 25 randomly selected patients with moderate to severe hypertension (diastolic pressure 105-130 mmHg), participated in the phenotyping study. The phenotyping was done by administering oral standard hydralazine dose of 150 mg/day in three divided doses. The 24 hour urinary MTP/hydralazine ratio was used to categorize patients into slow and fast acetylators. Of the patients studied 69.9% were slow acetylators while 30.4% were fast acetylators. The mean 24 hour urinary MTP/hydralazine ratio for slow acetylators was 1.01 +/- 0.95. This was significantly different from the fast acetylators where the mean 24 hour urinary MTP/hydralazine ratio was 10.6 +/- 4.4 (P < 0.001). The acetylator phenotyping divided the patients into two distinct populations and no further arbitrary method was required to divide the patients into either group.