From an original group of 229 patients with acute lymphoblastic leukemia or acute undifferentiated leukemia diagnosed in 1963 and 1964, 15 patients who achieved greater than 2 1/2 years continued complete remission were assigned either to continue with maintenance therapy or to discontinue therapy. Five of the 15 patients have died after surviving from six to ten years from the time of diagnosis. Of the remaining ten patients, all are alive and free of disease for more than 14 years from diagnosis. No relapses or deaths have occurred in patients off therapy for greater than five years. In this small series of patients, there was no difference in survival in the continued or discontinued therapy groups. No overt adverse effects of therapy have been seen in the ten surviving patients. These encouraging data need to be extended and confirmed, utilizing patients treated with the more sophisticated therapeutic approaches employed in modern protocols for ALL/AUL. Identification of adverse effects of therapy will require appropriate and careful surveillance of large groups of successfully treated patients.