DESCRIPTION (Applicant's Description): Although the cure rate of childhood ALL is among the highest of any cancers, there is still significant room for improvement both because some children still succumb to their disease and because others may obtain more treatment than is necessary for cure. Almost all children enter remission following initial therapy, but many patients harbor residual disease below the level that can be detected by conventional measurements, and there is a growing body of evidence that presence of this minimal residual disease may be an adverse prognostic factor. This project is designed to study a series of patients with B-precursor ALL enrolled on the Pediatric Oncology Group ALinC 17 pilot studies for newly diagnosed ALL in children. In this study we hypothesize that we will be able to distinguish leukemic cells from normal cells based on their abnormal phenotype using flow cytometry. In preliminary studies we have shown that we can detect leukemic cells at a level of up to 1/10,000 normal cells in virtually all cases of B-precursor ALL using a relatively simple four color flow cytometry assay. We will use this assay to determine how often cells with these phenotypes can be detected in patients in remission at the end of both induction therapy and consolidation therapy. In addition, we will determine if the presence of MRD at either of these times correlates with other clinical and laboratory risk factors or with the rate of relapse of leukemia. We recognize that the latter objective will not be answered definitively by this two year pilot study. If this pilot is successful, however, we will obtain a definitive answer from a subsequent study of over 2000 patients, and be able to design subsequent intervention studies to investigate if patients can benefit from specific therapy based on the presence of minimal residual disease.