Malignant lymphoma, diffuse, mixed small and large cell type, as defined in the Working Formulation, is heterogeneous both morphologically and immunophenotypically and, in some cases, clonality may be difficult to determine. Because gene rearrangement analysis has been shown to be a sensitive method for determining clonality, the authors genotyped 20 cases and compared the results with histologic and immunophenotypic findings. Immunophenotypic studies demonstrated that all lesions were composed predominantly of T cells. In addition, in eight cases either a monoclonal B-cell population (five lesions) or an aberrant immunophenotype (two T, one B) was detected, supporting a malignant diagnosis. In seven of these eight lymphomas, genotypic analysis confirmed the presence of a population of clonal cells. One case with an abnormal T-cell phenotype was germline. In 12 cases the immunophenotypic results were uncertain (i.e., no clonal population or abnormal immunophenotype was identified). Genotypic analysis provided evidence of clonality in eight. In four cases with uncertain immunophenotypic results, a clonal population also could not be identified with the use of Southern blot analysis. Thus, the authors conclude that gene rearrangement analysis is a valuable tool in the study of diffuse mixed cell lymphomas and is complementary to immunophenotypic studies. In addition, the authors analyzed the major breakpoint region of the bcl-2 protooncogene on chromosome 18, either by Southern blot analysis and/or with the polymerase chain reaction. The authors identified the t(14;18)(q32;q21) translocation in seven B-cell lymphomas, five of which were not considered to be of follicular center cell type on the basis of morphologic findings. These results suggest that the histologic spectrum of follicular center cell lymphomas is greater than is appreciated in the literature.