In a 14-year-old Japanese girl, manifested recurrent myalgia with elevated serum creatine kinase after moderate exercise became evident, and she was diagnosed as having a myopathic form of very-long chain acyl-CoA dehydrogenase deficiency. Her first clinical symptom of the disease was evident when she was 6 y of age. She had never had hypoglycemic attacks, and hepatomegaly and cardiomyopathy were absent. The diagnosis was suspected on the basis of the urinary organic acid profile after a 36-h fast, long-chain fatty acid-loading test, and the blood acylcarnitine profile. Acyl-CoA dehydrogenase activity with palmitoyl-CoA as a substrate was severely decreased in her fibroblasts, and the amount of very-long chain acyl-CoA dehydrogenase protein was reduced. She was a compound heterozygote of A416T from her father and R450H from her mother. Transient expression of mutant A416T cDNA retained a significant residual acyl-CoA dehydrogenase activity of 10% and 20% normal at 37°C and 30°C, respectively. Specific activity of A416T mutant protein was calculated to be one fifth that of control. In the case of R450H mutant expression, a low residual acyl-CoA dehydrogenase activity of 5% normal was detected at 30°C although significant activity was absent at 37°C. The R450H protein was not detected at 37°C but was clearly detected at one fourth the normal amount at 30°C. These results indicate that both mutations were temperature-sensitive mild mutations, the result being the mildest phenotype of very-long chain acylCoA dehydrogenase deficiency. VLCAD(EC 1.3.99.13; HUGO symbol, ACADVL) belongs to the -oxidation system of long-chain fatty acids together with mitochondrial enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase trifunctional protein (1, 2).The clinical phenotype of VLCAD deficiency (MIM# 201475) was first divided into two different forms (3); a severe presentation characterized by early onset of symptoms, with hypertrophic cardiomyopathy and a high incidence of death, and a milder form with a late onset with Reye-like disease, hypoketotic hypoglycemia, no cardiac involvement, and a more favorable outcome. Later occurrences in adolescents and adults have been reported, with the absence of hypoglycemia and cardiomyopathy but with episodes of rhabdomyolysis and myoglobinuria, usually triggered by exercise or fasting (4 -7). Hence VLCAD deficiency can be separated into three clinical phenotypes (8).Gene mutations in VLCAD deficiency have been reported, with a wide spectrum (8 -14). Andresen et al. (8) suggested a clear correlation of genotype with disease phenotype in VLCAD deficiency, from data on 58 independent mutations in 55 unrelated patients. They reported that most patients with the severe childhood form had two null mutations and that most