S-angiotensin-converting enzyme (SACE) was measured before and after 11 months (average) treatment in 29 patients with intrathoracic and 7 patients with extrathoracic tuberculosis. In intrathoracic tuberculosis there was a significant increase of mean SACE during treatment, from 22.6 ± 6.6 U/ml to 26.6 ± 4.5 U/ml, while mean SACE was unchanged in extrathoracic tuberculosis and in healthy untreated controls. SACE was significantly higher in untreated extrathoracic (29.9 ± 3.2 U/ml) than in intrathoracic tuberculosis. The results suggest that angiotensin-converting enzyme may be influenced by tuberculous infection, although the changes are small and within normal limits.