Five patients with Philadelphia-positive acute leukemia were cytogenetically and molecularly investigated in order to determine the localization of the breakpoints on chromosome 22. Rearrangements of the bcr segment were detected in one case with acute mixed leukemia in a child. Rearrangements in the BCR gene first intron, the so-called bcr2 and bcr3 regions, were detected in two other cases, one with an acute lymphoblastic leukemia (ALL) and one with mixed acute leukemia. No molecular rearrangement could be detected in the last two cases, an ALL and a T-cell acute lymphoblastic leukemia with a t(2;22) translocation.