Patients with aspartylglycosaminuria, a lysosomal storage disorder of glycoprotein degradation, express connective tissue signs that refer to impaired mechanical properties of the tissue. We studied the ultrastructure of the dermis of patients with aspartylglycosaminuria to detect possible alterations in the connective tissue matrix, alterations that could explain the clinical findings. The organization of fiber bundles was studied by light microscopy and scanning electron microscopy, and diameters and volume densities of individual collagen fibrils were measured. The histologic organization of the dermis in patients with aspartylglycosaminuria was normal. However, by scanning electron microscopy a looser organization and more irregular orientation of the fiber bundles were detected. Transmission electron microscopy revealed a strikingly abnormal variation in the diameters of individual collagen fibrils (from 20 to 160 nm) in all layers of the dermis, with slight irregularity of shape especially in the thickest fibrils. Occasional giant fibrils (greater than 200 nm) were observed. The distribution of the ruthenium red-positive material around the fibrils was normal. Ultrastructural changes similar to these have been found in the collagen fibrils of some patients with Ehlers-Danlos syndrome as well as certain other disorders affecting dermal connective tissue. Altered collagen fibril formation offers an explanation for the connective tissue lesions in aspartylglycosaminuria.