The Ph1 chromosome is present in 95% of patients with chronic myelogenous leukaemia (CML). The Ph1 chromosome also occurs in 5-25% of children and adults with acute lymphoblastic leukaemia (ALL). This observation raises questions as to whether these diseases are similar or identical. In patients with CML the c-abl and bcr genes are translocated and abnormally expressed. We studied molecular events related to bcr and c-abl in five patients with ALL to determine its relationship to CML. Four had the Ph1 chromosome; the fifth a probable Ph1 chromosome. c-abl and bcr abnormalities identical to CML were detected in four suggesting a common molecular basis. One patient with the Ph1 chromosome and c-abl translocation lacked these molecular changes but had abnormal c-abl gene transcription apparently unrelated to bcr. These data suggest that Ph1 chromosome positive ALL is heterogeneous; in some patients the molecular abnormality is identical to CML; in others c-abl is likewise involved but via a different mechanism.