The history of a forty year old patient is presented who was admitted with a clinical picture of chronic myeloid leukemia (CML). Laboratory findings, bone marrow morphology and molecular investigations supported this diagnosis, including b3/a2 as well as b2/a2 chimeric mRNA expression in support of a Philadelphia chromosome positive chronic myeloproliferation. In a fraction of the bone marrow content, however, an infiltrate different from that of CML could be seen. In addition, the morphology, cytochemistry, immunophenotyping and molecular analysis indicated that the coexisting neoplasia is hairy cell leukemia (HCL). Cell lineage specific interphase cytogenetic analysis proved a clonal relationship between the two neoplasias in a way that the HCL arose from one of the B-cells which, based on two cytogenetic markers, belonged to the original CML clone.