Background: Children with Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph-positive ALL) have a poor prognosis, and there is no consensus on the optimal treatment for this variant of ALL. Methods: We reviewed the medical records of patients with Ph-positive ALL who were treated with intensive chemotherapy, with or without bone marrow transplantation, by 10 study groups or large single institutions from 1986 to 1996. Data on 326 children and young adults, who ranged in age from 0.4 to 19.9 years (median, 8.1), were analyzed to determine the rate of complete remission and the probability of event-free, disease-free, and overall survival according to standard prognostic factors and type of treatment. Results: The 267 patients who had a complete remission after induction chemotherapy (82 percent) were stratified into three subgroups according to the age and leukocyte count at the time of diagnosis: those with the best prognosis (a leukocyte count of less than 50,000 per cubic millimeter and an age of less than 10 years; 95 patients); those with an intermediate prognosis (intermediate-risk features; 92 patients); and those with the worst prognosis (a leukocyte count of more than 100,000 per cubic millimeter; 80 patients). The estimates of disease-free survival at five years (±SE) were 49±5 percent (for patients with the best prognosis), 30±5 percent (for those with an intermediate prognosis), and 20±5 percent (for those with the worst prognosis) (P<0.001 for the overall comparison). We also found that transplantation of bone marrow from an HLA-matched related donor offered significantly greater benefit than intensive chemotherapy alone in terms of protecting patients from relapse or other adverse events (relative risk, 0.3; 95 percent confidence interval, 0.2 to 0.5; P<0.001). This finding was consistent in all three subgroups. Conclusions: Unlike the usual type of ALL, Ph-positive ALL is associated with a poor prognosis. Nevertheless, in some patients with favorable prognostic features, the disease can be controlled by intensive chemotherapy. Transplantation of bone marrow from an HLA-matched related donor is superior to other types of transplantation and to intensive chemotherapy alone in prolonging initial complete remissions. (C) 2000, Massachusetts Medical Society.