Various phenotypes of CPT II and VLACD deficiencies have been described. Severe neonatal or infantile clinical life-threatening symptoms may occur with hypoketotic hypoglycemia, liver failure, and cardiomyopathy during the first months or years of life (Demaugre et al, 1991; Bonnefont et al, 1996; Vianey-Saban et al. 1998; Andresen et al., 1999). Conversely, the "adult" forms of these diseases are more prevalent and have a clinical expression restricted to skeletal muscle. In the latter form, onset most often occurs in teenagers or young adults, and is mainly characterized by recurrent episodes of rhabdomyolysis triggered by prolonged exercise, fasting, cold or fever (DiMauro et al., 1973; Vianey-Saban et al. 1998). The major potential complication is acute renal failure following attacks of rhabdomyolysis.