A group of 227 cases of non-Hodgkin's lymphoma included seven favorable and four unfavorable histologic classes with collective median survival times of 83 and 16 months, respectively. The favorable group included three follicular subgroups (cleaved, mixed, and large noncleaved) and four diffuse classes (small lymphocytic, cleaved, Burkitt's noncleaved, and convoluted lymphocytic). The unfavorable group consisted of four diffuse subgroups (plasmacytoid lymphocytic, mixed, and small and large noncleaved). There were significant differences in collective median survivals between patients in Stage I--II and those in Stage III--IV in both the favorable group (not reached, NR versus 62 months, P less than 0.001) and the unfavorable group (57 months versus 12 months, P less than 0.01). The incidence of complete response to primary treatment was higher in the favorable than in the unfavorable group (75% versus 56%, P = 0.002) and in those with limited as compared with advanced disease. Complete responders in both prognostic groups had longer survival times than did partial or minimal responders. A significant difference in median complete remission duration was found between responders in Stage I--II versus Stages III-IV in the favorable group (not reached versus 40 months, P = 0.001) but not in the unfavorable one (56 months versus 22 months, P greater than 0.05). The absence of relapses after 41 months among complete responders in the favorable but not in the unfavorable group suggests a potential for cure in a proportion of cases from the former group. The incidence of complete response was lower and median remission duration was shorter after secondary as compared with primary treatment. The results of this study confirm the prognostic value of the Lukes and Collins classification system and the importance of initial staging and of achieving a complete response to primary treatment in both the favorable and unfavorable lymphomas.