Fanconi syndrome is an important presentation of respiratory chain disease. We report three patients who presented in the neonatal period with Fanconi syndrome, lactic acidosis and intrauterine growth retardation. In all three patients the major biochemical defect was in complex III of the mitochondrial respiratory chain, a relatively uncommon defect. The diagnosis could only be made by muscle biopsy as the defect was not expressed in cultured skin fibroblasts. Treatment with vitamins C and K3 and ubiquinone did not alter the course of the disease and all patients died before the age of 4 months.