The mucopolysaccharidoses are a group of inherited disorders of lysosomal storage of glycosaminoglycans. Among them, mucopolysaccharidosis (MPS) type II (Hunter's syndrome), caused by a deficiency in iduronate sulfatase, is the only one inherited in an X-linked recessive manner. We describe 12 Hunter's syndrome patients and seven carriers, with precise analysis of glycosaminoglycan content in urine and iduronate sulfatase activity in cultured fibroblasts and plasma. Their ages at the time of diagnosis ranged from 1 year 10 months to 11 years (mean 4.3 yr). The delay in diagnosis was from 1 month to 5 years (mean 2.1 yr) after the initial presentation. The most frequent initial complaints of the patients were delayed developmental milestones (75%) and speech (67%), although all patients were found to have coarsening of facial features at diagnosis. The difficulties in disease recognition allowed disease recurrence in four of the 11 families. Prompt clinical suspicion and referral will be important in genetic counseling for MPS type II and its management, if definitive therapy becomes available.