To detect chromosomal abnormalities in prodromal phase of adult T-cell leukemia (ATL), we established a clonal culture method for human T-lymphotropic virus type I (HTLV-I) infected T-cells in methylcellulose containing recombinant human interleukin 2 (rhIL-2). We tried to analyze chromosomes of 187 colonies (4, 23, 69, 74, and 17, from HTLV-I-uninfected normal T-cells, HTLV-I-Infected normal T-cells, HTLV-I carriers, smoldering ATL, and chronic ATL, respectively), using chromosomal banding methods. In the prodromal group, 53% of colonies (84/160) (36/69, 37/74, 11/17 in HTLV-I carriers, smoldering ATLs, and chronic ATL, respectively) had chromosomal abnormal clones. In HTLV-I carriers, multiple clones with simple chromosomal abnormalities were observed. In more progressed chronic ATL, more complex chromosomal abnormalities were detected, and specific colonies were selected. Thus, colonies in the prodromal phase of ATL are characterized by cytogenetical clonal evolution and clonal changes.