A case of peripheral T-cell lymphoma (PTL) occurring in a patient with Hodgkin's disease (HD) in relapse is described. The second neoplasm developed 25 months after the diagnosis of HD. Cytogenetic analysis on the lymph node biopsy at the time of diagnosis of PTL revealed the co-existence of two distinct, abnormal cell clones. The first clone was characterized by a reciprocal translocation t(5;7)(q13;q35) involving 7q35, namely the TCR-beta gene, as expected in T-cell lymphomas. The second cell clone carried trisomies for chromosomes 2, 5, 7, and 14. By immunophenotypic and molecular analysis as well as by in situ hybridization, it was possible to prove that the malignant T-cells and the Reed-Sternberg cells corresponded to different cell clones, one carrying the structural chromosome abnormalities and one carrying the numerical chromosome anomalies. These results indicate that the present case represented a true composite lymphoma.