6-Azauracil was studied in patients with "advanced" acute leukemia, refractory to standard chemotherapy. Under the conditions of the study this compound was found to have no significant antileukemic activity. As a result, these data have defined the natural history of this phase of acute leukemia. A significant proportion (30 per cent) of these patients showed objective hematologic improvement even in the absence of effective specific therapy, which might be ascribed to a prospective new agent. The median survival from the onset of the study of this group of patients was two months, and survival patterns for adults and children were similar.