Hypertension prevalence rates remain comparatively low in Nigeria, although the associated morbidity and mortality including that due to malignant hypertension (MHT) is considerable. To determine the factors that may be associated with the development of MHT we compared 74 patients with essential MHT (age 48 +/- 9 years, 59 male, blood pressure (BP) 234 +/- 31/140 +/- 17 mm Hg) with 74, age, gender and BP-matched patients with essential benign hypertension (BHT) (49 +/- 8 years, 60 male, 227 +/- 26/136 +/- 15 mm Hg). Body mass index was higher in the BHT [corrected] group by 1.3 (95% Cl: 0.5 to 2.1, P < 0.01). In the subset (25 MHT, 43 BHT) in whom hypertension had been diagnosed before presentation, duration of hypertension was shorter (P < 0.05) in the MHT group. Patients with MHT, were more likely to have been receiving inadequate therapy in the months before (OR 2.7, 95% Cl: 1.4 to 5.4), showed a decreasing proportion with increasing socio-economic class (chi2 = 5.79, P < 0.02) and had been exposed to a greater degree of stress (OR 3.5, 95% Cl: 1.7 to 7. 1). Smoking (OR 1.1, 0.6 to 2.3), alcohol use (OR 0.9, 0.5 to 1.8) and contraceptive pill use (OR 0.9, 0.1 to 8.6) did not impart excess risk. MHT is associated with the underprivileged and measures aimed at raising the general awareness and the socio-economic level of the people are expected to produce a decline in the incidence of MHT. Journal of Human Hypertension (2000) 14, 171-174.