Although hypertension is a complication of acute lymphoblastic leukemia (ALL), its true incidence in this disease is unknown. In this study the blood pressure profiles in all children newly diagnosed with ALL were reviewed over an 18-month period. Fourteen (46%) from a total of 30 patients were found to be hypertensive at presentation (n = 8) or during induction chemotherapy (n = 6). A patient with significant hypertension developed generalized convulsions; the rest were asymptomatic. Six patients were managed with antihypertensive drugs. Four patients with hypertension had renal enlargement on the initial ultrasound scan, which returned to normal when hematologic remission was achieved. One patient without hypertension had bilateral renal enlargement, but this persisted despite achieving remission. All patients with hypertension were normotensive at follow-up 2 to 18 months after induction chemotherapy. The presence of hypertension before therapy and its association with renal enlargement suggest that the leukemic process is an important etiologic factor. In all cases therapy aggravated or unmasked the elevation in blood pressure. Considering the high incidence of susceptible patients, increased awareness and prompt management may avoid possible life-threatening complications.