Malignant lymphomas--divided into Hodgkin's disease (HD) and non-Hodgkin lymphomas (NHL)--comprise a heterogeneous group of disorders. The normal counterparts of NHL tumour cells are various cells in the lymphocyte differentiation chain. The origin of Hodgkin and Reed-Sternberg cells is not elucidated, but it is today thought to possibly be a lymphoid cell. Myeloma stands morphologically close to NHL in that the tumour cell is a B lymphoid cell--sometimes there is a floating border between myeloma/plasmacytoma and immunocytoma (Kiel classification). The three entities have most serum markers in common, although to some extent of different usefulness for different entities. In this group of tumours, the main role of serum markers is not to act as a diagnostic tool, but to provide prognostic information and facilitate the detection of early relapses. In low grade NHL, serum markers can in addition aid in the decision on when to initiate therapy. New-comers in the serum marker business are the cytokines, some of which can now be detected in serum.