Non-Hodgkin's lymphomas (NHLs) are malignant tumours of the lymphoid system. Various risk factors have been described which are helpful in diagnosing, monitoring of the clinical course and predicting survival time of the patients. Proliferative activity of the tumour, measured by expression of Ki-67 antigen, is linked to the tumour proliferation rate and represents a recognised prognostic index in various tumours. In this study, the prognostic and predictive value of Ki-67 expression in NHL was evaluated.Expression of Ki-67 was examined using an immunohistochemical technique in archival paraffin-embedded sections taken from 56 previously untreated patients with diagnosed primary NHL. An attempt was made to test correlation between Ki-67 expression on one hand and clinical parameters of the patients and their survival on the other.Expression of Ki-67 antigen was noted in 75% of the tumour cases. In the group manifesting higher Ki-67 indices, survival of the patients was significantly shorter (p = 0.03). No significant correlation could be detected between Ki-67 antigen expression and clinical or pathological parameters of the patients.The results demonstrate that the most cases of NHL display the expression of Ki-67. Moreover, shortened survival was noted in patients with high expression of Ki-67.