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.