Adult T-cell leukaemia/lymphoma (ATLL) is a neoplasm of mature helper (CD4) T lymphocytes. Little is known, however, about the chromosome aberrations associated with the pathogenesis of this malignancy. Using molecular cytogenetic techniques we, therefore, investigated a 44-year-old man who had a 7-year history of ATLL with cutaneous involvement mimicking primary cutaneous T-cell lymphoma. Conventional cytogenetics revealed gross chromosomal changes with chromosome numbers ranging from 71 to 82. There were structural abnormalities of chromosomes 7 and 9, partial deletions of chromosomes 1, 3, 5 and 6, and loss of chromosomes 2, 4, 9, 11–14, 21 and 22. Multiplex-fluorescence in situ hybridization (M-FISH) identified two derivative chromosomes, der(6)t(6;7)(q16;q21) and der(7)t(6;7)(q16;q21)ins(6;12)(q2?;?), and a deletion of chromosome 1p. Conventional FISH confirmed the M-FISH findings. Comparative genomic hybridization of the blood revealed gains of DNA copy number at 1q12–25, 6p24–25, 9p23, 16p13–q13, 17q11–21, 19p13 and 20q13 and loss at 11p15 while lymph nodes showed gains at 3p22–24, 3q27–29, 7q36 and 15q26 and losses at 2p24–25, 2q37, 10p14–15, 11p15, 13q33–34 and 16p13.3. No DNA copy number changes were seen in a skin lesion. These results show the extent of genetic abnormalities within this malignancy.