Described are six patients with Alpers syndrome from four unrelated families. Affected individuals harbored the following combinations of POLG mutations: 1) A467T/W1020X, 2) W748S-E1143G/G848S, 3) A467T/A467T, and 4) A467T/G848S. Homozygosity for the A467T allele in one patient was associated with a later age at onset. Mitochondrial respiratory chain studies in skeletal muscle were normal in each case. Nine combinations of mutant POLG alleles that cause Alpers syndrome are summarized.