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Calcium metabolism was studied in 83 patients during eighteen months' rifampicin and isoniazid therapy for tuberculosis by measurements including calcium, alkaline phosphatase and 25-hydroxycholecalciferol (25-HCC). Five out of 52 Indian patients in the series were found to have osteomalacia, a prevalence probably no higher than in the Asian population in the UK at large. Moreover, osteomalacia responded to physiological supplementation with vitamin D. One European out of 31 had osteomalacia due to low vitamin D intake. Serum calcium was compared in 17 patients before and after six months of antituberculous chemotherapy but no significant difference was detected (P greater than 0.1). Two Indian patients were in positive calcium balance with low to normal plasma 25-HCC levels, indicating that an effect on 1,25 dihydroxyvitamin D activity during therapy was unlikely. It is concluded that rifampicin when combined with isoniazid has no significant effect on calcium metabolism over an eighteen-month treatment period.