Non-Hodgkin's lymphoma (NHL) consists of a heterogeneous group of disorders which have been difficult to study by epidemiological means in the past. However, recent advances in knowledge of the biology of NHL and improvements in its classification will greatly improve the quality of epidemiological studies in the future. Use of the Working Formulation and the current International Classification of Diseases for Oncology, along with paraffin immunohistochemistry, allow the delineation of NHL subgroups with possible etiological significance based on the biology of the disease. The collaboration of epidemiologists with expert pathologists in the design, performance, and evaluation of epidemiological studies of NHL is essential for such studies to be meaningful.