Although numerous publications have described the chromosome abnormalities in B-cell non-Hodgkin lymphoma and their significance, sparse literature exists pertaining to the chromosome abnormalities in T-cell lymphoma. We did cytogenetic analyses in 21 cases of peripheral T-cell lymphoma (PTCL). Chromosomally abnormal clones were identified in 15 (71%) of the cases, including 7 of the 10 cases in which the histologic distinction between a malignant and benign process was difficult. Abnormalities of chromosome 1 were observed in 10 cases; a breakpoint at 1p36 was demonstrated in 5 of these cases. Chromosome abnormalities previously attributed to B-cell malignancies were infrequent. These results suggest an association between 1p36 breakpoints and PTCL and emphasize the utility of cytogenetic analysis for documenting clonality among the histologically diverse groupings of PTCL.