An international multi-institutional clinicopathologic study of 1175 cases of non-Hodgkin's lymphoma sponsored by the National Institute has been completed. Histologic slides and clinical records were examined from previously untreated patients seen during the period between July 1971 and December 1975 at four institutions, three in the United States and one in Italy. The reproducibility and clinical relevance of the six major classifications of the non-Hodgkin's lymphomas was tested by six "expert" pathologists, each a proponent of a major classification, and six very experienced pathologists not identified with one of the major classifications. Immunologic methods were not employed in the study design. A summary of the methods employed and the conclusions of the study is described. The major conclusion was that all six classifications were valuable and comparable in reproducibility and clinical correlations. The clinical significance of a follicular architecture, independent of cell type was confirmed. A working formulation of non-Hodgkin's lymphomas is described which separates the disease into ten major types utilizing morphologic criteria only. Subtypes are also described which allow translation of all of the major classifications into comparable groups. Histologic criteria are presented for each major type and equivalent terms are given for each type in the six major classifications. The formulation is not proposed as a new classification but a means of translation among the various systems and to facilitate clinical comparisons of case reports and therapeutic trials. The report contains commentaries by five of the "expert" pathologists on the value and conclusions of this unique study.