Acute rejection of renal allografts was treated with the monoclonal antibody OKT3 given intravenously. A variety of adverse events were observed on days 1 and 2 following treatment with 5 mg/day OKT3 for 10 days including heart failure, pulmonary oedema and hypertension. Continuous monitoring of 19 patients treated with OKT3 for acute renal allograft rejection detected a transient increase, lasting 2 h, in systolic and diastolic blood pressures on day 1. A larger increase in systolic and diastolic pressures lasting 11-13 h was observed on day 2. Treatment with 5 mg OKT3 on day 3 did not significantly increase systolic or diastolic pressure. It is concluded that OKT3 can aggravate hypertension and hypertensive emergencies may be encountered during the initial phase of OKT3 treatment.