We reviewed 182 consecutive adult patients with low-grade malignancy, non-Hodgkin's lymphomas classified according to the Kiel classification, followed at the Division of Hematology, Policlinico S. Matteo, Pavia, from January 1975 to December 1981, to recognize, in each histopathologic type, important subgroups from the prognostic standpoint. Median follow-up was 36 months. No significant differences were observed in the response rate to conventional therapy (radiotherapy for localized disease, CVP for advanced stages) between the 4 cytologic types. The centrocytic-centroblastic lymphoma with diffuse nodal architecture showed an intermediate-grade malignancy (median survival, 50 months) and underwent cytologic progression to the high-grade malignancy centroblastic type in 10% of the cases. Large-cell centrocytic and polymorphic lymphoplasmacytoid lymphomas had a poor prognosis (median survival less than 30 months) when treated with conventional therapy for favorable histologies, and 6% of the cases transformed into the high-malignancy immunoblastic type. Patients with lymphocytic lymphoma with bulky mediastinum had a median survival of 20 months. The identification of these subgroups with a worse prognosis may have therapeutic implications.