To clarify predisposing factors for malignant hypertension, we retrospectively investigated the histories of 39 patients with malignant hypertension and 39 patients with benign hypertension. Between the malignant and benign groups, there was a statistically significant difference in blood pressure but not in age when hypertension was first noticed. The number of patients who had discontinued drug treatment was significantly greater in the malignant group (19; 49%) than in the benign group (11; 28%). Insufficient sleep, overwork, and/or mental burden of long duration were factors noticed within one year before the occurrence of the malignant phase in 11 (37%) of the 30 patients in that group in whom this information was available. Patients in the malignant group tended to belong to a lower social class. These results suggest that severe hypertension from an early phase, interruption of drug treatment, and physical and/or mental burden may predispose to the development of malignant hypertension, and that these predisposing factors are likely to be associated with social class.