In a 3-yr period, the Philadelphia (Ph1) was found in 4 of 43 children with acute lymphoblastic leukemia (ALL) in whom chromosomes were studied at diagnosis. The clinical, morphological, cytochemical, and immunologic findings in the Ph1-positive (Ph1+) cases were consistent with typical childhood ALL, indicating that identification of cases requires chromosome studies. A review of all reported cases of Ph1+ childhood ALL shows that Ph1+ patients are older and have higher initial platelet and white blood cell counts (WBC) than most children with ALL. However, a life table comparison between the reported cases of pH1+ ALL in children and randomly selected age-, sex-, and WBC-matched controls with ALL shows the duration on first marrow remission to be significantly shorter (p<0.02) for the Ph1+ cases. Ph1+ ALL is a distinct subtype of childhood ALL that is not rare and can be identified only by cytogenetic studies. The prognosis is poor. Cytogenetic studies should be done prospectively in a large group of children with ALL to define further this subgroup of patients and to confirm the findings of this retrospective analysis.