AbstractSix patients with lymphoproliferative disorder of large granular lymphocytes (LDGL) were studied for chromosomal abnormalities. When the patients were divided into two groups depending on the expression of a mature antigen of T cell lineage, CD3, two with CD3 + phenotype had a stable disease, whereas three of the four patients with CD3‐ phenotype had marked hepatosplenomegaly and a highly aggressive disease. Chromosomal abnormalities were detected in four of six patients, indicating a clonal proliferation of large granular lymphocytes in individual patients. In particular, chromosomal abnormalities were found in three of the four patients with CD3‐ LDGL. all of which had a germ‐line configuration of the T cell receptor beta‐chain gene. Thus, the chromosomal abnormalities provide definitive evidence for the clonal origin of the expanded cells in the C