An unusual malignant lymphoma with a partly nodular growth pattern was studied by morphologic, immunohistochemical, and DNA hybridization techniques. The nodular aggregates were composed of small and large, atypical lymphoid cells, and many histiocytic elements. On electron microscopy, both follicular dendritic reticulum cells and interdigitating reticulum cells were observed. Immunohistochemistry, using a broad panel of monoclonal antibodies, revealed a marked hyperplasia of dendritic reticulum cells in the nodular aggregates, and showed the atypical lymphoid cells to belong to the helper/inducer T-cell subset. The T-lineage of this nodular lymphoma was confirmed by demonstration of a clonal rearrangement of the T-cell receptor beta-chain genes. It is concluded that a nodular growth pattern caused by hyperplasia of follicular dendritic reticulum cells may occasionally be found in malignant lymphoma of the peripheral T-cell type.