A study of the prevalence of allergic patch test reactions to palladium chloride compared to nickel sulfate was performed in a group of Finnish schoolchildren. All adolescents 14–18 years of age in a Finnish town with 40 000 inhabitants, who had received orthodontic treatment with metallic appliances at a municipal dental clinic, were included in the study. The selection of patients was based on patient records. A non-treatment control group was randomly selected from the same age groups of the town population. A total of 700 subjects (77% of those invited), 417 (60%) girls and 283 (40%) boys, participated. The majority (91%) of the girls had pieced ears. Orthodontic treatment was equally common (67–70%) in the boys and the girls. The girls had a much higher frequency of allergic patch test reactions to both nickel sulfate and palladium chloride. Of the 700 adolescents tested 48 (7%) had an allergic patch test reaction to palladium chloride. Of the 417 girls, 44 (11%) were palladium-chloride-positive, whereas only 4 of the 283 boys tested (1%) had an allergic patch test reaction to palladium chloride 3 patients reacted to palladium chloride only, whereas all other patients with allergic patch test reactions to palladium chloride also had an allergic patch test reaction to nickel sulfate. The results support the concept of cross-reactivity between nickel sulfate and palladium chloride. The clinical significance of the allergic patch test reactions caused by palladium chloride remains unclear.