An apparently healthy girl aged 2 years 9 months developed a coma with hepatomegaly within 24 h after an influenza-like infection. Plasma glucose and urinary organic acid profile were normal but plasma and urinary carnitine concentrations were increased. Despite symptomatic therapy, she died 11 days later. Oxidation of [1-14C] palmitic acid in the patient's fibroblasts was severely decreased (13% of controls). Further investigations revealed a deficiency of carnitine palmitoyl transferase I (CPT I) in the patient's fibroblasts (15% of controls) whereas CPT II activity was normal. Only four patients with CPT I deficiency have been reported so far. The subtle clinical and biochemical presentation of this disorder, which may account for the small number of cases diagnosed, is discussed.