Abstract Malignant phase hypertension (MHT) represents the most severe form of hypertension, and many consider that this condition only occurs in poorly managed patients with previously known hypertension. To investigate this further, we studied 350 patients with MHT on the West Birmingham MHT database: 195 (55.7%) of these presented de novo, without any known past history of hypertension (Group 1), and 146 (41.7%) were previously known hypertensives (Group 2), of whom 86 were receiving antihypertensive therapy; in 9 patients, the status was uncertain. Median duration of clinical followup was similar in both groups (36.0 v 37.5 months, Mann-Whitney test P = .795). Patients presenting de novo with MHT (Group 1) were younger, with a predominance of whites and men. Nevertheless, the clinical features, blood pressures, and renal function at presentation were similar to MHT patients with previously known hypertension. Renal function at follow-up was also similar in both groups. There was an excess of women and nonwhites in MHT patients with previously known hypertension (Group 2), who also had higher mean follow-up blood pressures. On univariate life-table analysis, there was no statistically significant difference in survival time between Groups 1 and 2 (mean 57.5 v 63.5 months, median 36.0 v 37.0 months; log-rank test, P = .456). Using a multivariate Cox analysis of baseline variables, the independent predictors of outcome (death or dialysis) were age at presentation ( P = .0019), diastolic blood pressure ( P = .0466), serum urea ( P = .006), and serum creatinine ( P < .001). Whether the patient had presented de novo, without any known history of hypertension (Group 1) or had previously known hypertension (Group 2) did not independently predict outcome ( P = .6549). We suggest that MHT can occur de novo in patients without previously known hypertension, and the clinical characteristics and prognosis in such patients were similar to MHT patients with previously known hypertension.