Two hundred and thirty-four patients treated with continuous intravenous infusions of heparin were studied prospectively to seek a relation between the activated partial thromboplastin time (APTT) and recurrent venous thromboembolism or bleeding during treatment. One hundred and sixty-two patients were treated for venous thromboembolism and the remaining 72 for other diseases. The heparin dose was adjusted to keep the APTT between 1 1/2 and 2 1/2 times control levels. The five patients with venous thromboembolism in whom recurrence developed had a significantly lower APTT than patients without recurrence even though they received similar amounts of heparin. No recurrence developed in any patient with a mean APTT within the therapeutic range. Bleeding occurred in 19 patients whose mean heparin dose and APTT were similar to those of patients without bleeding. Thus, recurrence of venous thromboembolism during heparin treatment appears to be rare if the APTT is prolonged to 1 1/2 times or more control values at all times.