International Classification of Diseases for Oncology (ICD-O) code numbers for the non-Hodgkin's lymphomas (NHL) are assigned to the new Working Formulation (WF). This will facilitate the coding of NHL cases by registrars and pathologists by providing a ready means of translation of terms utilized in most of the existing classification schemes. Additional code numbers and synonyms are suggested to accommodate all of the terms in the six major classification systems currently in use and to permit the coding of immunologic cell type when appropriate tests have been performed. To demonstrate how the assignment of these code numbers can be useful, over 8000 incidence cases from the Surveillance, Epidemiology and End Results (SEER) Program (1977-1980) were coded according to the WF groups and the data analyzed. The prognostic significance of the three major groups in the WF is confirmed for each group in the SEER data and comparisons are discussed with the NCI study.