We present three patients, two children and one adult, with an unusual type of acute leukemia. Whereas the blast cells showed lymphoid morphology with correlating cytochemical staining, immunological phenotyping showed a pure myeloid in one and a biphenotypic (mixed lymphoid/myeloid) membrane marker profile in two of the patients. Cytogenetic studies revealed a 5q - chromosome as a common marker with additional individual changes. Two of the patients who were treated according to ALL-therapy protocols died without remission 5 and 4 weeks after diagnosis, respectively. Despite relapsing several times, the third patient survived for over 8 years. These three patients seem to represent one new subgroup of leukemias which can only be distinguished from typical ALL by determination of both cell surface markers and cytogenetic analysis.