Of 366 children with acute lymphoblastic leukemia (ALL) in whose bone marrow cells complete G-banding of chromosomes was successful, translocations were present at diagnosis in 141 (38.5%). The Philadelphia (Ph) chromosome was identified in 18 of these cases (4.9%). Features closely associated with the presence of the Ph chromosome were older age (median, 7.9 years), high leukocyte count (median, 47.3 X 10(9)/L), French-American-British L1 blast cell morphology, high incidence of CNS leukemia at diagnosis, and the common ALL immunophenotype. All patients were treated according to modern chemotherapeutic regimens for ALL used at our institution. Complete remissions were successfully induced in only 13 (72%) of the 18 patients with Ph + ALL, and only six remain free of leukemia for periods of 7+, 9+, 10+, 13+, 49+, and 70+ months. Our findings confirm the association of the Ph chromosome with classic high-risk features of ALL in children and suggest that this abnormality confers a very poor prognosis that has not yet been improved by modifications in established therapeutic regimens.