The aim of this study was to estimate the serum concentration of carbohydrate-deficient transferrin (CDT) in patients with rheumatoid arthritis (RA) and the relationship between the CDT level and disease activity in RA patients. Studies were carried out in 47 female patients with RA and 32 healthy women. Disease activity of RA was evaluated using the 28-joint count Disease Activity Score (DAS 28). Serum CDT was determined by particle-enhanced immunononephelometry using the N Latex CDT test. Patients with RA had significantly lower serum concentrations of CDT compared with controls. The correlation study showed the significant negative relationship between CDT and DAS 28 (r = - 0.483, p = 0.011). There were no correlations between serum CDT level and patient's age, disease duration, number of tender and swollen joints, and degree of disability evaluated by the Health Assessment Questionnaire. The level of CDT in patients with RA was significantly decreased and confirms the changes in transferrin glycosylation which are dependent on the disease activity. Therefore, measurement of CDT in the sera of patients with RA can be useful for the evaluation of disease activity in these patients.