The aim of the present study was to investigate the value of Ca 125, a tumour marker, in evaluation of pulmonary tuberculosis activity. This study included 96 subjects who were divided into three groups. Group 1 consisted of 40 patients with active pulmonary tuberculosis. Group 2 included 20 patients with inactive pulmonary tuberculosis. There were 36 healthy subjects in group 3. While measurement of serum Ca 125 level was performed only once in groups 2 and 3, Ca 125 levels were measured five times in group 1. The measurements were performed before the treatment, at the second, fourth and sixth months and the third year following the end of the treatment. Mean +/- SD serum Ca 125 concentrations were 109.7 +/- 86.9 U ml(-1) in group 1, 14.5 +/- 7.8 U ml(-1) in group 2 and 10.5 +/- 7.3 U ml(-1) in group 3. Serum Ca 125 levels were significantly higher in group 1 than in the other groups (P < 0.0001), but there was no significant statistical difference between the values of groups 2 and 3 (P > 0.05). Ca 125 levels in group 1 showed a significant decrease after treatment (P < 0.0001). For estimation of the activity of tuberculosis, the sensitivity and specificity of Ca 125 were found 97.5% and 100%, respectively at a 31 U ml(-1) cut-off point. Our results suggest that Ca 125 is beneficial in the determinaton of tuberculosis activity and in differentiation between active and inactive pulmonary tuberculosis.