We examined serum-free carnitine (SFC) concentrations and serum acylcarnitine (SAC)/SFC ratios in 40 severely handicapped patients, aged 2 to 36 years, and 69 age-matched control subjects. SFC levels in the patients treated with valproic acid (VPA) and/or receiving carnitine-deficient elemental diets (ED) were significantly lower, and their SAC/SFC ratios were significantly higher than in the other patients or in control subjects. There were 6 patients whose SFC levels were less than the -2SD level (15.8 +/- 6.7 microM, range 6.3-25.5) of those in control subjects (52.1 +/- 11.5 microM). They had no clinical symptoms of carnitine deficiency such as non-ketotic hypoglycemia, hepatomegaly, muscle weakness or cardiac function impairment, and showed normal transaminase, lipid and ammonia levels. In two cases (SFC = 11.0, 13.4 microM), the ketogenic responses to intravenous administration of fat-emulsion were impaired, but they were restored after D-,L-carnitine supplementation (30 mg/kg/day, po) for 1 month. However, in one case with the lowest SFC level (6.3 microM), the ketogenic responses to fat-emulsion infusion or fasting were normal, and dicarboxylic aciduria was not detected. These results indicate that 1) SFC levels are reduced in handicapped patients receiving VPA and/or ED, although clinical symptoms of carnitine deficiency do not easily develop, 2) some of these hypocarnitinemic cases show a subclinical impairment of hepatic fatty acid metabolism, not always correlated with the degree of SFC reduction, which can be restored by exogenous carnitine supplements, and therefore 3) in patients with acquired hypocarnitinemia, carnitine therapy should be considered, although a low SFC level alone may not imply an immediate indication.