An infant with moderate muscular hypotonia and congenital lactic acidosis died suddenly at the age of 3 months. Autopsy revealed no abnormalities responsible for this unexpected death. Measurement of mitochondrial enzymes involved in energy production indicated a severely decreased total pyruvate dehydrogenase complex (PDHC) activity in muscle tissue (0.23 nmoles x min-1 x mg protein-1, control range 2.8-8.7) and moderately decreased PDHC activity in fibroblasts (0.27 nmoles x min-1 x mg protein-1, control range 0.37-2.32). The activity of the first component E1 (pyruvate dehydrogenase) in muscle tissue was 10 times lower than that of controls (0.008 nmoles x min-1 x mg protein-1, control range 0.10-0.25). The activities of dihydrolipoyl dehydrogenase (E3) and various other mitochondrial enzymes were normal. Immunochemical analysis in skeletal muscle tissue and fibroblasts demonstrated a decrease in the amount of the alpha and beta subunits of E1. The features of this patient are compared with those of other patients reported in the literature with immunochemically confirmed combined E1 alpha and beta deficiency.