The human leukemic T-cell line Hut 78, derived from a patient suffering from Sézary syndrome and expressing a mature post-thymic membrane phenotype, shows a c-myc rearrangement beginning within 500 base pairs immediately 3′ to the c-myc exon 3. Chromosome analysis of the Hut 78 reveals the presence of a hyperdiploid karyotype with a large number of markers and rearrangements, though trisomy is the only cytogenetic anomaly involving chromosome 8. Moreover, as the abnormal c-myc appears to be duplicated, a duplication of the chromosome 8 carrying the abnormal c-myc probably occurred. Unlike four other human leukemic T-cell lines tested, the Hut 78 cells express a high amount of c-myc transcript, suggesting that the 3′ c-myc anomaly may cause a deregulation of the expression of this gene.