The cblC type of combined methylmalonic aciduria (MMA) and homocystinuria (HC) is the most common inborn error of vitamin B(12) metabolism and is caused by mutations in the MMACHC gene. To elucidate the spectrum of mutations that causes combined MMA and HC in Chinese patients, the MMACHC gene was sequenced in 79 unrelated Chinese patients. Sequence analysis identified 98.1% of disease alleles and found that all patients had at least one MMACHC mutation. A total of 24 mutations were identified. Out of the 24 mutations identified, 9 were novel ones, including missense mutations (c.365A>T and c.452A>G), nonsense mutations (c.315C>G and c.615C>A), deletions (c.99delA and c.277-3_c.303del30), duplications (c.248dupT and c.626dupT) and an insertion (c.445_446insA). The c.609G>A, c.658_660delAAG, c.482G>A, c.394C>T and c.80A>G mutations were the most common mutations and accounted for 80% of disease alleles. Haplotype analysis suggests that the spread of the c.80A>G, c.609G>A and c.658_660delAAG mutations in Chinese patients were caused by a founder effect. The results indicate that defects occurring in the MMACHC gene are the major cause of this disease in Chinese patients with combined MMA and HC, and direct mutation analysis can therefore be used as a rapid confirmatory diagnosis among these Chinese patients.