BACKGROUND: In case of tuberculous pneumonia, differentiation from bacterial lobar pneumonia is sometimes very difficult because clinical symptoms, signs and radiological images are very similar. So we investigated the usefulness of CA125, which is known to increase in tuberculous diseases, in differential diagnosis between tuberculous pneumonia (TBPN) and community acquired bacterial lobar pneumonia (LP). METHODS: Serum CA125 level was measured in 20 patients with TBPN (female 12 male 8: mean age 36.1 years) and 14 patients with LP (female 5 male 9: mean age 45.1 years) by radioimmunoassay (Centocor(R) CA125 RIA kit). RESULTS: 1) The serum CA125 level in TBPN (333.7 283.5 u/ml) was higher than in LP (60.9 66.2 u/ml). (P < 0.05) 2) If we took cut-off value as 195 u/ml in differential diagnosis between TBPN and LP, the sensitivity and specificity of CA125 level in the diagnosis of TBPN were 70% and 93%, respectively. 3) There was no significant difference in serum CA125 level between noncavitary TBPN (242.1 +/- 16.6 u/ml,n=10) and cavitary TBPN (399.6 +/- 318.4 u/ml,n=10). (P > 0.05) 4) Following up of serum CA125 level after initiation of antituberculosis treatment showed rapid decline and approach to near normal range in 6 months. CONCLUSION: High serum CA125 level (> 195 u/ml) was useful in differential diagnosis of TBPN from LP.