The nm23-H1 gene is a putative metastasis-suppressor gene encoding a 17 kDa protein with nucleoside diphosphate kinase activity. Expression of nm23-H1/NDPK-A correlates inversely with the metastasising potential of some human tumours and experimental animal cells. No nm23 expression studies exist for human malignant lymphomas so far. In this study, we examined nm23-H1 expression by Northern and immunohistochemical analysis in 106 primary lymphoma samples from patients with Hodgkin's disease (HD) (n = 15), high-grade non-Hodgkin's lymphoma (NHL) from different lineages (n = 71) and low-grade NHL (n = 20). Both inter- and intra-subtype variations in nm23-H1/NDPK-A expression levels were demonstrated by all disease subtypes. Besides this heterogeneity, a general trend towards highly malignant samples expressing higher nm23-H1/NDPK-A, levels than the low-grade lymphomas was observed. Both adult and childhood HD and high-grade NHL samples exhibited significantly higher NDPK-A expression than the low-grade NHL found only in adults. High nm23-H1/NDPK-A levels in lymphoma samples did not always reflect proliferative activity of tumour cells as monitored by Ki-67 antigen staining. Fifty samples were further investigated for possible mutations in the nm23-H1 coding sequence by means of reverse transcriptase-polymerase chain reaction (RT-PCR) and single-strand conformation polymorphism (SSCP) analysis. No mutation was found by this screening. Our results suggest a role for nm23-H1 expression in the disease aggressiveness of lymphomas.