Axillary skin biopsies from fifteen patients with axillary hyperhidrosis who had received long-term treatment with aqueous aluminium chloride solution were examined histologically. The apocrine glands were normal, but the eccrine glands showed conspicuous morphological changes of varying severity. These included vacuolization of the secretory epithelium, dilatation of eccrine acini with atrophy of secretory cells, and accumulation of PAS-positive, diastase-resistant material in the dilated lumen of the secretory coils. The histological changes correlated in most patients with decreased secretion of sweat. We conclude that long-term blockage of the distal acrosyringium due to aluminium salts may lead to functional and structural degeneration of the eccrine acini.