Late relapse with nodular lymphoma after treatment for diffuse non-Hodgkin's lymphoma. Histologic conversion from a low-grade non-Hodgkin's lymphoma (NHL) into a more aggressive histologic pattern is a common, well-documented event in NHL. The converse phenomenon, appearance of a low-grade, follicular NHL after treatment for diffuse, intermediate, or high-grade NHL, has only recently been recognized. The clinical, morphologic, and immunologic features of a patient in whom relapse with an indolent nodular lymphoma was noticed after combination chemotherapy for diffuse lymphoma are presented. Immunologic markers at presentation and relapse were similar. Other previously reported cases are reviewed. Implications for diagnosis and therapy as well as the pathogenesis of this unique form of conversion are discussed. 