<jats:title>Abstract</jats:title><jats:p>A diagnosis of 3‐methylglutaconic aciduria type I (OMIM: 250950) based on elevated urinary excretion of 3‐methylglutaconic acid (3MGA), 3‐methylglutaric acid (3MG) and 3‐hydroxyisovaleric acid (3HIVA) was made in a 61‐year‐old female patient presenting with leukoencephalopathy slowly progressing over more than 30 years. The diagnosis was confirmed at the enzymatic and molecular level. <jats:italic>In vivo</jats:italic> brain MR spectroscopic imaging (MRSI) was performed at 3.0 T, and one‐dimensional and two‐dimensional <jats:italic>in vitro</jats:italic> NMR spectroscopy of body fluids of the patient was performed at 11.7 T. Additionally, we measured 1D <jats:sup>1</jats:sup>H‐NMR spectra of urine of seven patients with a total of four different inborn errors of leucine metabolism. Increased concentrations of 3HIVA, 3MGA (<jats:italic>cis</jats:italic> and <jats:italic>trans</jats:italic>) and 3MG were observed in the NMR spectra of the patient's urine. In the cerebrospinal fluid, the 3HIVA concentration was 10 times higher than in the plasma of the patient and only the <jats:italic>cis</jats:italic> isomer of 3MGA was observed. <jats:italic>In vivo</jats:italic> brain MRSI showed an abnormal resonance at 1.28 ppm that may be caused by 3HIVA. Comparison of <jats:sup>1</jats:sup>H‐NMR spectra of urine samples from all eight patients studied, representing five different inborn errors of leucine metabolism, showed that each disease has typical NMR characteristics. Our leukoencephalopathy patient suffers from a late‐onset form of 3‐methylglutaconic aciduria type I. In the literature, only very few adult patients with this conditions have been described, and 3HIVA accumulation in white matter in the brain has not been presented before in these patients. Our data demonstrate that <jats:sup>1</jats:sup>H‐NMR spectroscopy of urine can easily discriminate between the known inborn errors of leucine metabolism and provide the correct diagnosis. Copyright © 2006 John Wiley & Sons, Ltd.