We describe respiratory chain complex IV deficiency (cytochrome c oxidase deficiency) in a female infant with a neonatal rapidly progressive fatal course characterized by microcephaly, encephalopathy, persistent lactic acidosis, and hypertrophic cardiomyopathy. Postmortem cardiac muscle study showed marked complex IV deficiency. In contrast, complex IV activity was only slightly decreased in the skeletal muscle. Subsequent molecular investigations showed compound heterozygosity for two known pathogenic mutations in the COX15 gene. We compare the findings in our patient to those of the three previously reported cases. © 2011 Wiley-Liss, Inc.