The safety and efficacy of diazoxide administered intravenously in the treatment of children with acute severe hypertension have been evaluated by a collaborative study. Observations of the response of blood pressure in 36 patients, ranging in age from two months to 18 years, during the initial episode of hospitalization reveal diazoxide treatment to be effective in lowering blood pressure in 94 per cent of the cases. No serious adverse circulatory, fluid and electrolyte, metabolic or hematologic effects were observed. Symptomatic and subjective reactions observed with diazoxide administered intravenously to children were identical with those described in adults. Reinstitution of other means of antihypertensive therapy is safe and effective when delayed until the transiently induced period of hypotension has passed. Repeated use of diazoxide for subsequent recurrence of severe hypertension was equally effective and safe in 93 per cent of the instances. The results lead us to recommend the use of intravenous diazoxide for treatment of children with severe symptomatic hypertension especially when it is refractory to control by other hypertensive agents.