Methylmalonic acidemia (MMA) is an inborn error of organic acid metabolism that occurs in infancy with hypotonia, vomiting, dehydration, lethargy and failure to thrive and is biochemically characterized by metabolic ketoacidosis, hyperammonemia and sometimes hyperglycinemia. It results from deficiency of methylmalonyl-CoA mutase activity due to a defect in the mutase apoenzyme or to deficient function of one of the enzymes required for metabolism of its cofactor vitamin B12. Tubulointerstitial nephritis with progressive impairment of renal function is one of the most frequent long-term complications. We describe a case of a 17-year-old girl with methylmalonic acidemia unresponsive to vitamin B12 therapy. The clinical symptoms appeared at 4 months of life. She progressed into end stage renal disease and in January 1996 she started on hemodialytic treatment. In November 1996 we performed a kidney transplant. At present, urinary excretion of methylmalonic acid is normal and the renal function of the transplanted kidney is normal without any rejection episodes. We think that a kidney transplant could be a good therapeutic choice for the metabolic alterations in MMA with end stage renal disease. Indeed it would seem that the small methylmalonyl-CoA mutase activity present in the transplanted kidney could be sufficient to ensure normal metabolism of organic acids. Otherwise, the therapeutic goal can be achieved with a protein-restricted diet.