In this study we analyzed proliferative activity of myeloma cells and a possible correlation with selected clinical data, histological features and survival in 59 patients with newly diagnosed multiple myeloma (27 females and 32 males, mean age 62 years). Imunohistochemical method was applied using Ki-67 antibody on B5-fixed and paraffin-embedded bone marrow specimens to evaluate growth fraction of myeloma cells. Clinical staging was done according to the Durie-Salmon classification (4 patients had stage I disease, 16 patients stage II and 39 patients stage III). The number of Ki-67+ myeloma cells ranged from 1% to 36% (mean value 7%). In 39 of 59 patients (66.1%) number of Ki-67+ cells was less than 10% (cases with low proliferative index). Ki-67 expression significantly correlated with the clinical stage, beta2-microglobulin level, plasma cell morphology, volume of myeloma infiltration and the extent of osteolytic lesions. Patients with increased proliferative index (Ki-67+ cells > or = 10%) showed a significantly shorter survival compared to those with low proliferative index (14 months vs. 36 months, p = 0.023). However, this difference was not shown in multivariate analysis, particularly due to the high correlation between proliferative activity and plasma cell morphology and the volume of myeloma infiltration.