The Pediatric Oncology Group institutions initiated extensive subclassification of cases of acute lymphocytic leukemia (ALL) at diagnosis into laboratory-designated categories. Included was a French-American-British (FAB) classification of all new patients, which was reviewed by a central six-member committee. In addition, on the basis of immunologic criteria, patients were defined as having T-, B-, pre-B-, or "null" cell leukemia. Slides from 617 patients were reviewed. Five hundred forty-six (88.5%) were classified as L1, 51 (8.3%) were classified as L2, 9 (1.5%) were classified as L3, and the remainder could not be assigned. Concordance within the committee was good: in 71% of the cases the committee was unanimous, and in an additional 17% only one member disagreed. In only 11 cases (1.8%) was diagreement such that a majority classification could not be assigned. Institutions assigned L2 more frequently. There was a strong correlation with L3 for B-cell disease only. However, four patients had unequivocal B-cell disease and unmistakable L1 morphologic type, whereas one and had L3 morphologic features and had non-B-cell disease. There was no correlation between the other immunologic markers or periodic acid-Schiff stain and FAB classification, nor between L1 or L2 and risk factors. However, for the 248 null cell and pre-B-cell patients, L2 was more frequent among patients in the poor-risk group (P = 0.008). The time to first failure was significantly shorter for patients with L3 morphologic type. The induction failure rate of L2 patients was significantly greater than that of L1 patients (P = 0.016). With analysis of the duration of remission and adjustment for risk factors, the impact of L2 morphologic characteristics on outcome was not significant (P = 0.18) in null cell patients. Even unadjusted for risk factors, there was no impact of L2 morphologic type on outcome in the pre-B-cell phenotype. It can be concluded that other risk factors overshadow the impact of L1 and L2 morphologic features in predicting duration of remission.