Sjögren-Larsson syndrome (SLS) is an autosomal recessively inherited neurocutaneous disorder caused by a deficiency of the microsomal enzyme fatty aldehyde dehydrogenase (FALDH). We report the clinical characteristics and the results of molecular studies in 19 SLS patients. Patients 1-17 show the classical triad of severe clinical abnormalities including ichthyosis, mental retardation and spasticity. Most patients were born preterm, and all patients exhibit ocular abnormalities and pruritus. Electro-encephalography shows a slow background activity, without other abnormalities. MRI of the brain shows an arrest of myelination, periventricular signal abnormalities of white matter and mild ventricular enlargement. Cerebral 1H-MR spectroscopy reveals a characteristic, abnormal lipid peak. The degree of white matter abnormality in the MRIs and the height of the lipid peak in 1H-MR spectra do not correlate with the severity of the neurological signs. The clinical presentation and the clinical course is strikingly similar in these patients. Patient 18 shows a mild phenotype that essentially contains the same, but less severe, clinical features. Patient 19 exhibits the typical, but very mild, dermatological and ocular abnormalities, without any clinical neurological involvement. The diagnosis of SLS was confirmed by demonstration of the enzyme defect in cultured skin fibroblasts. Furthermore, as might be predicted from the essential role of FALDH in leucotriene B4 (LTB4) metabolism, elevated urinary concentrations of LTB4 and 20-OH-LTB4 were found in all patients studied. Molecular studies of the FALDH gene revealed eight different mutations, including three new ones: a large 26-base pair deletion (21-46del), a missense mutation (80C T) and an insertion mutation (487-488insA). The vast majority of SLS patients seem to be severely affected independent of their genotype.