We investigated the presence of circulating immune complexes (CICs) in serum from tuberculosis (TB) patients with different degrees of pulmonary involvement. Patients were classified into four groups according to the extent of lung involvement: mild (N = 9), moderate (N = 12), moderate plus (N = 16), and severe cases (N = 10). A search for CICs by the 3.5% PEG precipitation test showed that the CIC values of patients with the moderate plus or severe form of pulmonary TB were significantly higher compared to healthy controls and to mild and moderate cases (P < 0.01 and P < 0.001, respectively). Further analysis demonstrated that increased CIC levels were associated with increased autoantibody production, since this abnormality was more prevalent in patients with advanced disease (P < 0.01), who also showed a significant reduction of CD4+ T lymphocytes. The immunoregulatory and pathogenetic implications of these findings are discussed.