Serum calcium was prospectively studied in 50 consecutive patients with active pulmonary tuberculosis. Twenty-four of them (48%) developed hypercalcaemia during an observation period of at least 8 weeks. Maximal increase in serum calcium (corrected for serum albumin) occurred three weeks after initiation of treatment, by which time 28% of the patients were hypercalcaemic. The increase in serum calcium was followed by a spontaneous remission. Only two patients developed symptoms related to hypercalcaemia, which promptly responded to steroid administration. No patient received vitamin D supplements before or during the study. No correlation could be found between hypercalcaemia and either the presence of acid-fast bacilli in the sputum or the season of the year. There was a trend for higher serum calcium values in the patients with the more severe radiographic changes on admission. Hypercalcaemia in patients with pulmonary tuberculosis seems to be triggered by chemotherapy. However, the mechanism(s) by which anti-tuberculosis treatment affects calcium metabolism remains uncertain.