Eve Roman and Alexandra

Introduction Epidemiology is the basic quantitative science of public health and, as such, is concerned with the distribution, determinants, treatment, management, and potential control of disease. Concentrating on the first two of these, this chapter reviews the epidemiology of lymphomas – a heterogeneous group of malignancies that is estimated to account for around 3–4% of cancers worldwide. Epidemiological reports on lymphomas often begin, and sometimes end, by stating that little is known about the causes of the cancers under study. This is slowly changing, as evidence about the pathological diversity of the various lymphoma subtypes accumulates. At present, however, the issue of lymphoma classification continues to permeate much of the literature, since, in order to originate and test hypotheses about pathogenesis, it is vitally important to describe accurately and understand underlying descriptive disease patterns. Implicit in this is the need to use appropriate disease classifications; it is this requirement that has beleaguered epidemiological research into the lymphoid malignancies. In short, the classification of hematological malignancies has changed markedly over recent decades, and will continue to do so as biological understanding increases and new diagnostic methods and techniques are developed. One problem for epidemiological research concerns the use of historical classifications emanating from the latter half of the nineteenth century – long before there were any effective treatments or real understanding of the relationships between lymphoid malignancies, the normal bone marrow and immune system, and before anything was known about the cellular and genetic basis of malignant transformation. The application of modern disease classifications is, however, now beginning to discriminate between subtypes, revealing many features that future etiological hypotheses will undoubtedly seek to address. Accordingly, the next few decades promise to be an exciting time for epidemiological research into lymphoid, as well as other hematological, malignancies.