Normal fibroblasts exposed to N-acetylmannosamine yielded lysosome-rich granular fractions loaded with free (unbound) sialic acid, whose velocity of egress increased with increasing initial loading. Fibroblast granular fractions of patients with Salla disease exhibited negligible egress of sialic acid, whether endogenous or derived from N-acetylmannosamine exposure. Salla disease represents the first disorder demonstrated to be caused by defective transport of a monosaccharide out of cellular lysosomes.