A male infant presented at 5 months of age with vomiting, developmental stagnation and convulsions. Complex I activity was in skeletal muscle 0.025 mU/mU CS (N 0.044-0.265) and in fibroblasts 0.046 mU/mU CS (N 0.100-0.307). Despite riboflavine supplementation progressive neurological deterioration occurred and he died at 14 months of age. During the mother's following pregnancy complex I activity was measured in chorionic villi and found mildly reduced, pregnancy was continued. A male infant was born who presented at 7 months of age with vomiting, developmental stagnation and hypotonia. Complex I activity was in skeletal muscle 0.031 mU/mU CS and in fibroblasts 0.100 mU/mU CS. There was progressive neurological deterioration and he died at 17 months of age. Complex I activity in autopsy liver of both patients was normal. Apparently, complex I deficiency presenting in infancy can have a fatal outcome despite only mild reduction of enzyme activity in skeletal muscle and/or fibroblasts, and chorionic villi and normal activity in liver.