Chromosome aberrations and clinical features of three patients with adult T cell leukemia-lymphoma (ATL) not associated with human T cell leukemia virus type I (HTLV-I) are described. From their clinical features, two patients were diagnosed as acute type and one patient was diagnosed as chronic type, which later converted to acute crisis. Clonal and many chromosomal abnormalities were observed before therapy in the two acute type cases and at relapse in the chronic type case. Karyotype aberrations, including trisomy 3, trisomy 7, trisomy 21, del(6)(q21), del(10)(p13), 14q11 translocation, and loss of X chromosome, all of which are frequently found in HTLV-I associated ATL, were also seen in these cases of HTLV-I-negative ATL.