In a group of 111 subjects with multiple myeloma (MM) comprising a group of 34 patients examined when the diagnosis was established and a group of 77 patients examined in different stages of development of MM the authors evaluated the relationship between interleukin-6 (IL-6) serum levels and the clinical activity and the stage of the disease. In both groups a significant relationship was found between IL-6 and the clinical activity of MM; "stable" and "active" stages of the disease differed by the frequency of elevated values and the level of IL-6. In both groups the authors recorded rising levels and a rising rate of subjects with elevated IL-6 levels with advancing stages of the disease. When staging systems were used according to Durie-Salmon, the British Medical Research Council and according to Bataille the highest IL-6 values were recorded in the third stage of the disease, these values being significantly higher than in stages 1 and 2. In the group assembled at the time of assessment of the diagnosis of MM the described differences did not reach (with the exception of the evaluation according to Bataille) statistical significance. The classification of patients in stages 1-3 into sub-groups with regard to the activity of the disease ("stable" and "active") was associated with significantly different IL-6 levels. The investigation revealed that examination of IL-6 levels contributes at present rather to the understanding of the pathogenesis and biology of MM than to practical evaluation of the severity, activity and stage of the disease.