Pretreatment blast cells from 739 adults with acute lymphoblastic leukemia (ALL) were immunophenotyped as part of a prospective treatment protocol study. Among 192 patients (26%) with T lineage ALL, 47 (6%; 24% of T lineage ALL) had lymphoblasts without sheep erythrocyte rosette formation, but with pan-T antigen CD7 on the membrane and intracellular CD3 proteins mostly in perinuclear accumulation. The T-cell surface antigens CD5 and/or CD2 and focal acid phosphatase were additional markers of this subgroup traditionally called pre-T ALL, whereas thymocyte antigen CD1 as well as CD4 and CD8 antigens were not expressed. Hematopoietic progenitor cell markers, namely terminal deoxynucleotidyl transferase (TdT), and in part common ALL antigen (CD10), HLA-DR antigens, and/or My-10 (CD34), a unique antigen of marrow cells absent in thymus cells, further characterized this immature T-ALL form of putative prothymocytic phenotype (CD7+/intracellular CD3+/TdT+/My-10+/-/HLA-DR+/-/CD10+/-). The prethymic T cell character was supported by germ-line T-cell receptor beta genes found in 21 of 36 patients analyzed. In five cases only T gamma-chain genes were rearranged. Fifteen patients, however, had rearrangements of both T beta and T gamma genes. Immunoglobulin heavy chain genes were rearranged only in two cases. Pre-T ALL differed significantly from E-rosette+ T-ALL in some presenting clinical features, namely mediastinal mass, lymphoadenopathy, and platelet count, and independently of clinical factors in prognosis (P = .02, median remission duration: 15.7 v 33.5 months, and P = .02, median survival time: 24.6 v 50.7 months). We conclude that ALL classification based solely on T- or B-cell lineage affiliation is not sufficient but needs further subdivision according to relevant maturation stages as exemplified here within the T-cell axis. The putative prethymic T cell progenitor phenotype described might help elucidate the sequence of genetic events that commit normal hematopoietic cells to the T-cell lineage.