In a retrospective study of 301 non-Hodgkin's lymphoma patients the clinical and prognostic value of the classification of Lukes and Collins was studied. A slight majority of the cases were of the follicular center cell types. Stages III-IV were found in 46% of the entire group at the time of diagnosis. The lymphomas showing follicular center cell types were frequently localized (63%). About 65% of all cases survived one year and about 45% five years. The females showed a better survival. The nodal lymphoma cases had the poorest survival. Two prognostically different subgroups of non-Hodgkin's lymphomas were discerned. The more favourable group consisted of the small cleaved FCC type (five-year relative survival 74%), the small lymphocytic type (62%) and the plasmacytoid lymphocytic type (51%). The small non-cleaved FCC type, the large non-cleaved FCC type and the immunoblastic sarcoma type formed the other group with a less favourable prognosis. Mortality due to these types manifested itself to a level of 65% in the first two years.