A 7-year-old Japanese boy with Ph1-positive-lymphoblastic lymphoma is described. The diagnosis was based on biopsied tonsils which were enlarged at the time of admission. On the eighth day after admission an enlarged mediastinal mass was detected on a chest X-ray film. The lymphoblasts which appeared in the peripheral blood and bone marrow proved to be T-cells. Chromosome studies on the bone marrow cells revealed two abnormal cell lines; one had a 7;11 translocation and the other a 7;11 translocation and a 9;22 translocation, forming the Ph1-chromosome. The latter line with the Ph1-chromosome was considered to have been derived from the former line without the Ph1. Our findings show that the Ph1-chromosome may be a secondary change in the course of karyotypic evolution.