AbstractWe report on two new patients with straight‐chain acyl‐coenzyme A oxidase deficiency. Early onset hypotonia, seizures and psychomotor delay were observed in both cases. Plasma very‐long‐chain fatty acids were abnormal in both patients, whereas the plasma levels of phytanic acid, pristanic acid, the bile acid intermediates DHCA and THCA, and erythrocyte plasmalogen levels were normal. Studies in fibroblasts from the two patients revealed a deficiency of one of the two peroxisomal acyl‐CoA oxidases, that is, straight‐chain acyl‐CoA oxidase (ACOX1). Subsequent molecular analysis of ACOX1 showed a homozygous deletion, which removes a large part of intron 3 and exons 4–14 in the first patient. Mutation analysis in the second patient revealed compound heterozygosity for two mutations, including: (1) a c.692 G > T (p.G231V) mutation and (2) skipping of exon 13 (c.1729_1935del (p.G577_E645del). © 2008 Wiley‐Liss, Inc.