OBJECTIVE: To test for the hypothesis Rifampicin causes hypocalcaemia with hyperparathyroidism. Our recent discovery. STUDY DESIGN: Experimental study. PATIENTS AND METHODS: Antituberculous chemotherapy was suspected of causing hypocalcaemia, when tetany was observed in two patients on such therapy for spinal tuberculosis. A resemblance was found in the literature between Rifampicin and Phenytoin a drug known to cause hypocalcaemia) regarding hepatic enzyme induction. Twenty one patients randomly selected were included in this study, which did not have any known major factor predisposing to hypocalcaemia. In every case, serum calcium and parathyroid hormone levels were done before and thirty days after stopping Rifampicin and compared. RESULTS: Serum calcium increased by a mean value of 0.76 mg/dl and serum parathyroid hormone decreased by a mean of 25.2pglml during thirty days after stopping Rifampicin. CONCLUSION: Rifampicin causes hypocalcaemia with hyperparathyroidism (Possible effects of prolonged hypocalcaemia during treatment of spinal tuberculosis are discussed in detail.)