Report is summary of the results of study designed to ascertain the significance of soluble CD138 (sCD138) assessment in patients with different monoclonal gammopathies. Previous studies have shown that sCD138 is shed from the surface of myeloma cells into serum and that this marker is a new independent prognostic parameter in multiple myeloma. In presented study was evaluated serum sCD138 level in 14 patients with monoclonal gammopathy of undetermined significance (MGUS) and in 17 patients with multiple myeloma (MM), all MM patients were treated by high-dose chemotherapy regimen with subsequent autologous transplantation of peripheral blood stem cells. To determine the sCD138 level we used a rapid and simple ELISA procedure. The mean serum sCD138 level of patients with MGUS was 32 ng/ml (range: 5-128). Soluble CD138 levels were elevated in the sera of 10 out of 17 (59%) multiple myeloma patients, the mean baseline sCD138 concentration was 1542 ng/ml (range: 10-17300). In spite of small number of patients the difference between MGUS and MM group was highly statistically significant (p<0.001). Multiple myeloma patients with high level of sCD138 at diagnosis (cut-off value: 500 ng/ml) had worse prognosis despite of good response to chemotherapy in some of them (p=0.029). It seems that determination of sCD138 can be recommended as a helpful and reliable marker for differential diagnosis as well as prognosis of monoclonal gammopathies.