Methylmalonic acidemia (MMA) is an autosomal recessive inborn error of metabolism caused by inadequate function of methylmalonyl-CoA mutase. We studied five Korean patients diagnosed with mut MMA, here, we report five new missense mutations (G94E, R369C, S344Y, N189K, and T230I) and a previously reported mutation (R369H) that, this is the first time this mutation has been identified in Korean individuals. Genetic heterogeneity in mut MMA is high. The R369H mutation has been identified in America and Japan. To date, more than 55 different mutations have been identified in mut MMA. A majority of mutations is novel with only three (G717V, E117X, and N219Y) being reported more frequently, the G717V mutation was found in Africa-Americans and Ghanaian. The E117X mutation has been found in Japan. The N219Y mutation has been found in Caucasians and Arab. The R369H mutation is the first mutation identified in three nations (Korea, Japan, and America).