A patient is described with chronic myelogenous leukemia (CML) and an unusual karyotype 46XY,t(Y;12) (q11;p12), whose clinical course was complicated by T‐cell lymphoma 5 years later. At that time bone marrow cells showed an additional karyotypic abnormality 46XY,t(Y;12) (q11;p12) del(7) (pter‐p21), which remained unchanged until blastic transformation of the CML 8 months later. The bone marrow biopsy specimen, which revealed the blastic transformation of the CML, also showed evidence for localization of T‐cell lymphoma. This case, added to two previously reported cases of the concurrence of CML and T‐cell lymphoma, suggests a relationship between the two diseases, which is discussed. Copyright © 1990 American Cancer Society