Aspartylglucosaminuria (McKusick 208400) is a lysosomopathy associated with aspartylglucosaminidase (L-aspartamido-beta-N-acetylglucosamine amidohydrolase, EC 18.104.22.168) deficiency. It has been most frequently encountered in Finland, where the regional incidence may be as high as 1 in 3600 births. In North America it is very rare, having been reported in only 8 patients. We encountered 4 patients with aspartylglucosaminuria in a Canadian family of 12 siblings. The 4 siblings affected--2 brothers and 2 sisters--were apparently normal at birth; however, their developmental milestones, particularly speech, were slow, and they acquired only a simple vocabulary. Throughout life, there was a progressive coarsening of facial features; 3 had inguinal hernia and recurrent diarrhea; all became severely retarded and by the 4th decade showed evident deterioration of both cognitive and motor skills; 2 exhibited cyclical behavioural changes. Three of the siblings have died, at 33, 39 and 44 years of age. Two died of bronchopneumonia and 1 of asphyxiation following aspiration. In the urine of all 4 siblings, and in the 1 liver examined, we found 2-acetamido-1-N-(4-L-aspartyl)-2-deoxy-beta-D-glucosamine (GlcNAc-Asn) and alpha-D-mannose-(1,6)-beta-D-mannose-(1,4)-2-acetamido- 2-deoxy-beta-D-glucose-(1,4)-2-acetamido-1-N-(4-L-aspartyl)-2-deoxy-beta - D-glucosamine (Man2-GlcNAc2-Asn). Compared with the level of activity in controls, aspartylglucosaminidase activity was less than 2% in fibroblasts from 3 of the siblings, less than 0.5% in leukocytes from 1 sibling, and less than 1% in the liver of 1 sibling, whereas other acid hydrolase activities in these tissues were normal. Ultrastructural studies of skin showed that fibroblasts, endothelial cells and pericytes contained vacuoles with fine reticulo-floccular material. Glial and neuronal cells of the central nervous system showed similar inclusions as well as others composed of concentric or parallel membranous arrays intermingled with lipid droplets.