This presentation deals with the gross and microscopic pathology of HD. Recent advances in immunohistochemistry, gene rearrangement studies and cell culture are not discussed, except where they shed light on the pathology. Clinical and pathological experience, over the past 2 decades, suggests that HD should be divided into six subtypes, as originally proposed by Lukes et al. (1966b), rather than the four subtypes included in the Rye classification. Nodular lymphocyte/histiocyte predominant HD forms a clinicopathological entity separate from the other subtypes. It most frequently presents at a single nodal site and, even without therapy, progresses only slowly over a period of many years. A proportion of the patients (in the region of 10%) develop large cell NHL and a smaller number develop other types of Hodgkin's disease. This progression is not due to therapy since it most frequently occurs in untreated patients. Characteristic polylobated RS cell variants are seen in NLPHD. These differ from classic RS cells in that they have a B-cell phenotype, they do not show light chain restriction and, therefore, they do not appear to be a clonal proliferation. Although current dogma states that classic RS cells must be identified before a diagnosis of HD, including NLPHD, is made, it is the author's contention, supported by immunohistochemistry, that this type of RS cell does not occur in NLPHD. Polylobated RS cell variants in the appropriate cellular setting are, in themselves, diagnostic of NLPHD. They also serve to differentiate NLPHD from progressive transformation of germinal centres, an unusual proliferative expansion that may occur in association with HD but which, in itself, appears to be an entirely benign, reactive process. Diffuse lymphocyte/histiocyte predominant HD (DLPHD) differs from NLPHD in its diffuse growth pattern and the frequent presence of larger numbers of histiocytes. Polylobated RS cells are characteristic of both diseases. In some biopsies nodular and diffuse areas are seen in the same lymph node. Despite these similarities, the two diseases differ clinically (NLPHD is usually stage 1, DLPHD is frequently of a higher stage) and in their immunohistochemistry (fewer RS cell variants in DLPHD contain J-chain than in NLPHD). The reasons for these differences are not apparent and require further investigation. It may be due, at least in part, to the inclusion of cases of MCHD and NHL in the DLPHD category. Nodular sclerosis is the commonest category of HD in most reported series.(ABSTRACT TRUNCATED AT 400 WORDS)