In a group of 70 subjects with multiple myeloma (MM) formed by 25 patients examined before establishment of the diagnosis and 45 patients evaluated in different developmental stages of the disease, the authors evaluated the relationship between the value of the bromodeoxyuridine "labelling index" (BrdUrd-LI) and the clinical activity, stage and course of the disease. The authors revealed statistically significantly higher values of BrdUrd-LI in the group of patients in the "active" phase of the disease than in the "stable-plateau" phase of the disease. This applies also to findings within different clinical stages of the disease. The authors did not reveal any relationship of BrdUrd-LI values and the stage of progress of the disease evaluated by means of staging systems according to Durie-Salmon, the British Medical Research Council and Bataille. Patients in the "stationary-plateau" phase with a longer than five-year duration of the disease (61-211 months) had all low BrdUrd-LI values (median 1.4%), while subjects in the "late-preterminal" stage of MM with possible extramedullary spread and leukaemization of the process had in all instances higher values (3.2%). Evidence was provided that examination of the proliferating characteristics of myeloma plasmocytes by means of BrdUrd-LI contributes to a better evaluation of the severity, development and prognosis of the disease.