Urinary sialoglycoconjugates were studied in 22 patients with inherited deficiency of 1-aspartamido-beta-N-acetylglucosamine amidohydrolase (aspartylglycosaminuria), in eight obligate heterozygotes, and in age- and sex-matched control subjects. Total sialic acid excretion was significantly higher in the patients (38.3 +/- 17.7 mumol/mmol creatinine, mean +/- S.D.) than in the matched controls (17.7 +/- 7.3 mumol/mmol creatinine, p less than 0.001). The sialic acid output in the heterozygotes did not differ from that of the controls. Gel filtration studies revealed that the increase in urinary sialic acid in aspartylglycosaminuria is of bound type and confined to the low molecular mass region. A linear positive correlation was found between the output of sialic acid and glycoasparagine in the individual patients (r = 0.77, p less than 0.001). The amount of sialylated metabolites excreted in urine did not correlate with the severity of clinical manifestations in aspartyl-glycosaminuria.