Clinical, radiological and biochemical findings are described in a male newborn with mucolipidosis II (I-cell disease). At birth a characteristic somatic picture, skeletal anomalies, vacuolated lymphocytes in peripheral blood, typical signs of a metabolic disorder, were present. The daily excretion of urinary mucopolysaccharides was within normal range, nevertheless an anomalous pattern consisting in a high amount of dermatan sulfate was found. The excretion of urinary sialyl oligosaccharidases was increased. Cultured skin fibroblasts showed the typical "I-cell" phenomenon. The activities of serveral lysosomal enzymes were strikingly increased in serum, but not in leucocytes. No significant variations of acid hydrolase activities were found in the patient's relatives.