The prevalence of nickel allergy (sensitization) and the associations with ear piercing, use of dental braces and hand eczema were assessed in a cohort of 1,501 8th grade schoolchildren (aged 12-16 years) in Odense, Denmark. Nickel allergy was found in 8.6% and was clinically relevant in 69% of cases. Nickel allergy was found most frequently in girls and the association with ear piercing was confirmed. Application of dental braces (oral nickel exposure) prior to ear piercing (cutaneous nickel exposure) was associated with a significantly reduced prevalence of nickel allergy. In adolescents a significant association was found between hand eczema and nickel allergy. A follow-up study of this population is planned in order to assess the course and development of contact dermatitis, hand eczema and atopic diseases in adulthood and after choice of occupation.