We investigated the surface markers on lymphoblasts from 37 patients with acute lymphoblastic leukemia. Spontaneous rosette formation with sheep erythrocytes (E rosettes) identified T cells and the presence of surface immunoglobulin identified B cells. Eight patients had T-marker lymphoblasts; 28 had no markers (null lymphoblasts), and one patient had B-marker lymphoblasts. The eight patients with T-marker acute lymphoblastic leukemia had massive leukemic infiltration, frequently a mediastinal mass, and a poor prognosis. The T-marker lymphoblasts had a weak or negative periodic acid-Schiff reaction and a very low antigenic stimulation to allogeneic lymphocytes. The association of T-marker lymphoblasts and some of the conventional poor-risk factors appears to be reliable in predicting a poor prognosis.