W e studied metabolic, polypeptide and genetic variation in eight glutaric acidemia type II (GA II) patients with electron transfer flavoprotein (ETF) deficiency. As measured by 3H-fatty acid oxidations in fibroblasts, beta-oxidation pathway flux correlated well with clinical phenotypes. In six patients with severe neonatal onset GA II, oxidation of [9,10(n)-3H]-palmitate ranged from 2% to 22% of control and of [9,10(n)-3H]myristate, from 2% to 26% of control. Of two patients with late onset GA II, one had intermediate residual activities with these substrates and the other normal activities. Radiolabeling and immunoprecipitation studies revealed that three of the six neonatal onset GA II patients had greatly diminished or absent alpha- and beta-ETF subunits, consistent with a failure to assemble a stable heterodimer. Another neonatal onset patient showed normal synthesis of beta-ETF but decreased synthesis of alpha-ETF. Two neonatal onset and two late onset GA II patients showed normal synthesis of both subunits. Analysis of the pre-alpha-ETF coding sequence revealed seven different mutations in the six patients with neonatal onset GA II. The most common mutation was a methionine for threonine substitution at codon 266 found in four unrelated patients, while all the other mutations were seen in single patients. No mutations were detected in the two patients with late onset GA II.