Background: According to studies on adults, patch testing with aluminium chloride hexahydrate 2% pet. is insufficient to detect aluminium allergy, and a 10% preparation is recommended. Other studies suggest that a 2% preparation is sufficient for testing children. Objectives: To review three previously published Swedish studies on patch testing children with aluminium chloride hexahydrate 2% pet. Patients/Methods: Altogether, 601 children with persistent itching subcutaneous nodules (granulomas) induced by aluminium-adsorbed vaccines were patch tested with aluminium chloride hexahydrate 2% pet. and metallic aluminium in (a) a pertussis vaccine trial, (b) clinical practice, and (ca) prospective study. Results: Overall, 459 children had positive reactions to the 2% pet. preparation. Another 10 reacted positively only to metallic aluminium. An extreme positive reaction (+++) was seen in 65% of children aged 1 to 2 years as compared with 22% of children aged 7 years. From 8 years onwards, extreme positive reactions were scarce. Conclusions: Aluminium chloride hexahydrate 2% pet. is sufficient to trace aluminium allergy in children. Small children are at risk of extreme reactions. We thus suggest that aluminium chloride hexahydrate 10% pet. should not be used routinely in children before the age of 7 to 8 years.