A series of 9 infants, ranging in age from 3 months to 5 years (average: 2 years), suffered from idiopathic myopathic carnitine deficiency presenting as hypotonia and motor delay. Secondary carnitine deficiency was eliminated by appropriate tests. Muscle carnitine concentration ranged from 2.3-7.1 nmol/mg non-collagen protein (NCP; average: 4.87 nmol/mg NCP; normal: 22 +/- 6 nmol/mg NCP). Lipid accumulation in muscle was observed in 2 of 8 patients. Therapy with L-carnitine (100 mg/kg/day in most patients) was given with clinical and laboratory follow-up 6 months later. In 7 of 9 patients, muscle tone and motor function improved. Muscle carnitine concentration increased to a range of 2.7-23.4 nmol/mg (average: 12.27 nmol/mg). In some patients the muscle carnitine content multiplied by a factor of 3-4, but carnitine concentration reached the normal range in only 2 patients. Most infants with idiopathic carnitine deficiency did benefit from 6 months of therapy; however, in order to achieve full recovery the duration of therapy should probably continue for longer periods, with a dose of not less than 100 mg/kg/day.