Malignant lymphomas have traditionally been classified as either Hodgkin's (HL) or non-Hodgkin's lymphomas (NHL). The tumor cells in NHL have been identified as being derived from T- or B-lymphocytes, but not until recently was it possible to identify the origin of the tumor cells in HL. Differential diagnostic problems may exist with morphological overlaps between HL and NHL but do not reflect an underlying biological relationship. Recent observations indicate that HL and NHL may be more closely related than previously believed. HL and NHL may occur sequentially or simultaneously in one and the same individual and in a few cases a clonal relationship between the lymphomas has been proven. It is thus possible that a biological relationship between the disorders may exist in addition to morphological overlaps, and the purpose with this review is to summarize the different relations between HL and NHL.