OBJECTIVE Combined methylmalonic aciduria (MMA) and homocystinuria, cblC type, is the most common MMA in Mainland China. Its clinical spectrum varies from severe neonatal-onset forms with brain injury and high mortality to milder forms with late onset. Timely diagnosis and adequate treatment greatly improve the prognosis. In the past 15 years, many Chinese patients with this condition have achieved favorable treatment outcomes, and some of them have reached childbearing age. Here, we report the first case of Chinese woman with cblC undergoing a successful pregnancy and delivering a healthy boy. METHODS A patient of late-onset cblC, who achieved successful pregnancy and delivery of a healthy boy, was enrolled in our studies. The patient and her disease characteristics were analyzed, including age at onset, age at diagnosis, clinical presentation, clinical classification, family history, laboratory findings and MMACHC gene mutation. RESULTS Our patient presented mild neurological symptoms at the age of 15 years. She had the compound heterozygous mutations, c.315C>G and c.482G>A, on the MMACHC gene. After long-term treatment with cobalamin, calcium folinate, l-carnitine and betaine, along with normal diet, she recovered completely. At age 23, she visited us for genetic counseling and fetal evaluation at 15 weeks of gestation. Her general condition and the fetal growth were normal. At 20 weeks of gestation, intramuscular cobalamin was changed to pure hydroxocobalamin (1 mg, every other day) because of markedly elevated urine MMA and plasma total homocysteine. In addition, the dosages of l-carnitine (1 g, p.o., tds), folic acid (5 mg, p.o., tds) and betaine (1 g, p.o., tds) were increased. Protein intake was not restricted. CONCLUSIONS This is the first report of a Chinese woman with cblC, undergoing a successful pregnancy and delivering a healthy baby at term. The favorable outcome of the patient and her fetus should owe much to the mild phenotype of her disease.