A method is described for the detection of abnormal oligosaccharides in a small (5 ml) volume of urine, employing filtration on a Bio Gel P-6 column, determination of neutral sugar and bound sialic acid, and determination of creatinine content. With this method increased urinary excretion of sialic acid-rich oligosaccharides has been detected in nine patients with mucolipidoses (five cases of mucolipidosis II and four patients of mucolipidosis, with beta-galactosidase deficiency). The filtration patterns of oligosaccharides in mucolipidoses were clearly distinguishable from those in other inborn errors of metabolism. Total excreted oligosaccharides were increased 5--30-fold in these patients; mucolipidosis II, 640--1350 microgram neutral sugar/mg creatinine; control 54 +/- 20 microgram neutral sugar/mg creatinine. The oligosaccharides consisted of three sialic acid-rich fractions and were common in both types of mucolipidosis. Our data indicate that hypersialyoligosacchariduria is the main biochemical feature of both types of mucolipidosis.