A patient with Philadelphia chromosome (Ph1) positive chronic myelogenous leukemia (CML) entered a blast crisis localized to lymph nodes. On light microscopy, by morphology and histochemical staining, the blasts were undifferentiated. In spite of terminal deoxynucleotidyl transferase positivity, some of the lymph node cells expressed a myeloid differentiation antigen, OKM1, and were peroxidase positive by transmission electron microscopy (TEM). However, the majority of cells were peroxidase negative on TEM and expressed OKT-10, a marker found on both primitive myeloid and lymphoid cells. Cultures of lymph node cells stimulated with Epstein-Barr virus or lipopolysaccharide (LPS) revealed the Ph1, indicating B cell involvement in the CML. T cells from cultures stimulated with L4-phytohemagglutinin and T cell growth factor were negative for the Ph1. In unstimulated lymph node cells, the uncomplicated Ph1 could not be demonstrated; instead, a unique complex karyotype involving a masked Ph1 was identified in these and the LPS cultures. This karyotype was not found in bone marrow (BM) metaphase cells. Instead, BM cells showed either the simple Ph1 or the Ph1 with a rearrangement involving chromosomes 13 and 20. The patient had transient responses to three chemotherapy regimens, two of which were designed to treat acute lymphocytic leukemia, but he died 8 months after disease acceleration without BM blast crisis. These findings are compatible with an extramedullary blast crisis originating in a primitive cell with both myeloid and lymphoid characteristics.