Calcium levels were determined in sera of patients suffering from various lung diseases. Healthy volunteers served as controls. Significant differences were found between the serum calcium levels of patients with active lung tuberculosis and those of controls (P less than 0.01). After treatment, serum calcium levels decrease to normal values in these patients. It was also found that there were significant differences in serum calcium levels of patients with primary lung carcinoma (P less than 0.01) and of patients with metastatic lung carcinoma as compared to controls (P less than 0.01). On the other hand, normal serum calcium levels were found in patients with pulmonary diseases with or without an infection. In conclusion, it seems likely that a combination of mechanisms plays a role in the pathogenesis of hypercalcaemia in pulmonary tuberculosis and primary and metastatic lung carcinoma.