Two siblings presented with a new phenotype consisting of fatal progressive macrocephaly and hypertrophic cardiomyopathy. Onset of symptoms started in both patients at the end of the first month of life with massive brain swelling causing macrocephaly and evolving to extensive brain destruction. Light microscopy of the lesions showed extensive small‐vessel proliferation and gliosis. A distinct deficiency of complex I of mitochondrial respiratory chain was established in cultured fibroblasts, skeletal muscle, and heart muscle. Specific lack of complex I protein was demonstrated by two‐dimensional gel electrophoresis.