Methylmalonic aciduria is a common organic aciduria disease. Recently, gas chromatography-mass spectrometry has been used to diagnose methylmalonic aciduria in China. Often, however, the diagnosis of methylmalonic aciduria is delayed because of a lack of technical expertise and the limited experience of general clinicians in China. In this study, the natural history, clinical features, and outcome of 77 Chinese patients with methylmalonic aciduria were investigated. Of the 77 patients, 31 (40.3%) had isolated methylmalonic aciduria and 46 (59.7%) had methylmalonic aciduria combined with homocystinemia. Thus, we observed a higher rate of the combined disease than studies conducted in other countries, suggesting that it might be more common in China. Total plasma homocysteine measurement might enable differential diagnoses of methylmalonic aciduria to be distinguished. The clinical spectrum of these 77 patients with methylmalonic aciduria ranged from neonatal death and severe symptoms to benign asymptomatic organic aciduria. Neonatal and infantile onset, which was a characteristic of the majority of cases, was associated with a greater severity relative to later-onset cases. Among the 17 cases who had onset after 3 years of age, only 1 patient had isolated methylmalonic aciduria and 16 had combined methylmalonic aciduria and homocystinemia. Nine of the patients with combined methylmalonic aciduria and homocystinemia completely recovered and exhibited normal intelligence, whereas seven improved, with a mild handicap.