Chromosomes of 30 patients with adult T-cell leukemia were analyzed. Chromosome abnormalities were found in all the patients examined. The modal chromosome number of abnormal cells was hypodiploid in 2 patients, diploid in 14, and hyperdiploid in 9. The remaining 5 patients had bimodal chromosome numbers (diploid and hyperdiploid modes). Although all the patients showed various numerical or structural chromosome abnormalities, they also had common chromosome abnormalities. Aberrations of chromosome 1 were noted in 20 of the 30 patients, aberrations of chromosome 3 were seen in 20, trisomy 6 or 6q-was found in 17, aberrations of chromosome 10 were noted in 16, aberrations of the long arm of chromosome 14 were seen in 9, and trisomy 18 was seen in 7. There was no particular relationship between the difference in clinical symptoms and disparity in chromosome abnormalities.