We describe a male infant of Austrian ancestry, the main clinical features including attacks of dyspnea due to laryngomalacia, severe mental and motor retardation, pronounced splenohepatomegaly and vacuolisation of peripheral lymphocytes. The clinical condition deteriorated progressively and the child died at the age of 21 months. Phase and electron microscopy of cultured skin fibroblasts showed multiple vacuoles and inclusions suggestive of a lysosomal storage disorder. Increased excretion of free sialic acid was demonstrated by resorcinol staining after routine thin-layer screening for urinary oligosaccharides. Quantitative analyses of urine, leucocytes and cultured fibroblasts revealed 10 to 30-fold increased concentration of free sialic acid. In addition, 3-fold elevated amounts of sialyloligosaccharides were found in the urine. The activities of a variety of lysosomal enzymes, including sialidase, were normal. Our case is compared with infantile sialic acid storage disease recently observed by other authors. The close convergence of clinical, morphological and biochemical signs support the concept of a distinct lysosomal disease entity. The basic defect is so far unknown.