Methylmalonic acidaemia (MMA) is a heterogeneous group of rare genetic metabolic disorders caused by defects related to intracellular cobalamin (vitamin B(12)) metabolism. Increasing evidence has emerged suggesting that free radical generation is involved in the pathophysiology of neurodegenerative diseases, including some inborn errors of metabolism. We have previously identified in MMA patients several differentially expressed proteins involved in oxidative stress [mitochondrial superoxide dismutase (MnSOD) and mitochondrial glycerophosphate dehydrogenase (mGPDH)] and apoptosis by a proteomic approach. We have now extensively evaluated various parameters related to oxidative stress and apoptosis in cultured fibroblasts from a spectrum of patients with methylmalonic acidaemia. Fibroblasts from several MMA patients showed a significant increase in intracellular reactive oxygen species (ROS) content and in MnSOD expression level with respect to controls, suggesting a cellular response to intrinsic ROS stress. Moreover, we have demonstrated, using siRNA, that mGPDH is an important ROS generator in MMA patients. Cells from patients with MMA had a higher rate of apoptosis than those of controls and there was evidence that this process primarily involves the mitochondrial/caspase-dependent pathway. ROS level-phenotype correlation revealed that patients with severe neonatal cblB disorder had elevated intracellular ROS content. These findings support the possible role of oxidative stress in the pathophysiology of methylmalonic acidaemia.