We ascertained the incidence of hypercalcemia in 79 consecutive patients with active pulmonary tuberculosis and a control group of 79 patients with chronic obstructive pulmonary disease. Twenty-two patients developed hypercalcemia (serum calcium greater than 10.5 mg/dl) within 4 to 16 weeks after initiation of chemotherapy for tuberculosis. The duration of hypercalcemia ranged from 1 to 7 months, and remission occurred spontaneously in all patients. The mean daily vitamin D supplement was greater in hypercalcemic patients than in the normocalcemic group. There was a positive correlation between daily vitamin D supplement and degree and duration of hypercalcemia. Mean serum calcium in patients with tuberculosis was higher than in patients with chronic obstructive pulmonary disease supplemented with the same dose of vitamin D. Hypercalcemia appears to be related to the activity of pulmonary tuberculosis and the intake of vitamin D; the exact mechanism, however, remains unknown.