Early phase hyperglycemia, associated with increased rates of insulin and C-peptide secretion after oral administration of 100 g glucose, was observed among patients with pulmonary tuberculosis who were taking rifampicin. This early phase hyperglycemia appeared shortly after rifampicin was started and it disappeared completely a few days after rifampicin was discontinued. No difference in oral glucose tolerance was noted between healthy normal subjects and patients with pulmonary tuberculosis who were not taking any medications. Antituberculous drugs other than rifampicin did not induce early phase hyperglycemia. Because intravenous glucose tolerance was normal in patients treated with rifampicin, it is suggested that rifampicin produces an early phase hyperglycemia possibly by augmenting intestinal absorption of glucose.