Summary Background. It was recently shown that filaggrin null mutation carrier status was associated with nickel allergy and self-reported intolerance to costume jewellery. Because of the biochemical characteristics of filaggrin, it may show nickel barrier properties in the stratum corneum. Objectives. To investigate whether subjects with filaggrin null mutations report nickel dermatitis at an earlier age than wild-type individuals, and to analyse whether null mutation carriers have stronger patch test reactivity to nickel sulfate than do wild-type individuals. Materials. A total of 3471 Danes (18-69 years of age) answered a questionnaire about general health, and underwent patch testing and filaggrin genotyping. Results. The mean number of years at risk of developing nickel dermatitis was significantly lower for the filaggrin null genotype than for the wild-type genotype when ear piercing status was considered. In positive patch test readings, the proportion of null mutants increased with increasing reaction strength. Conclusions. Filaggrin null mutations may lower the age of onset of nickel dermatitis. The hypothesis that ear piercings obscure the effect of filaggrin null mutations on the development of nickel allergy in statistical analyses was supported. An association between the null genotype and increased nickel sensitivity was indicated by patch test reading and questionnaire data.