Factors affecting the degree of blood pressure control achieved by antihypertensive drug treatment in 150 patients with moderate and severe essential hypertension were analyzed. Patients were under continuing treatment for 1-23 years. Most patients were referred for hypertension which was difficult to control. All were managed by the senior author. They received multiple drug regimens and extensive efforts were made to encourage their adherence to the regimens. In spite of these efforts, 16 (11%) had poor blood pressure control. Satisfactory control was achieved in 52 (35%) patients and excellent control in 82 (55%) patients. As a group, their ideal body weight (IBW) was 125 +/- 22% (mean +/- SD) of normal. The mean number of antihypertensive tablets per day was significantly greater (P = 0.0009) in those with poor control as compared to those with satisfactory and excellent control. As % IBW increased, the number of antihypertensive tablets increased (P = 0.0021). We examined the relationship of blood pressure control with an index of compliance, with psychosocial factors (life events score, martial status) and with socioeconomic factors (work status, income). Poor blood pressure control was associated with a lower compliance index (P less than 0.0001) and a higher life events score (P less than 0.006). Poorly controlled patients were more likely to have poor health in general (P = 0.0002), to have kept fewer medical appointments during the preceding year (P less than 0.0001), to be unmarried (P less than 0.0001), to be unemployed (P less than 0.0028) and have a lower income (P less than 0.009). As % IBW increased, compliance index decreased, (P = 0.0045). Therefore, psychosocial and socioeconomic factors, as well as physical factors, influence blood pressure control in moderate and severe essential hypertension.