A follow-up study of 202 children who had received hyposensitization with aluminium-containing allergens showed that 1-3 years after cessation of hyposensitization 13 children still had severely pruiginous treatment-resistant subcutaneous nodules in their forearms. Because of their long persistence the nodules of six children were studied in detail. Histologically, the nodules showed infiltration with lymphocytes (forming germinal centres), macrophages, plasma cells, mast cells and a few eosinophils. In five patients aluminium crystals were found scattered between the cells and, in addition, the phagosomes of the macrophages contained aluminium. Patch tests for aluminium were positive in four of the six patients. It is concluded that persisting nodules during hyposensitization with aluminium-containing allergens may indicate development of aluminium hypersensitivity, and if this is confirmed hyposensitization should be discontinued.