The metabolic profiles of three patients with fatal infantile mitochondrial myopathy with de Toni-Fanconi-Debré syndrome were studied by simultaneous analysis, after urease treatment of urinary organic acids, carbohydrates, polyols and amino acids using gas chromatography/mass spectrometry (GC/MS). All three patients persistently showed lactic aciduria, phosphaturia, glucosuria and generalized amino aciduria. This abnormal urinary metabolic profile was observed before the onset of any clinical symptoms, indicating that chemical diagnosis may be done presymptomatically. In one patient, the concentration of lactate increased in parallel with the severity of the clinical condition, whereas the urinary levels of 3-hydroxybutyrate, amino acids and glucose fluctuated and showed only a general tendency to increase with the clinical course. The above results suggest that simultaneous GC/MS analyses, without fractionation, of urinary metabolites facilitate not only the early chemical diagnosis either before or after the first onset, but also follow-up studies, providing an important index for the evaluation of the severity and clinical course in patients with this disorder.