Background In 1995, we established a cohort of 1501 unselected eighth-grade schoolchildren to investigate the course of nickel allergy into adult life. Objectives To follow the course of nickel allergy and clinically relevant nickel dermatitis over 15 years from adolescence to adulthood, and the effect of ear piercing, atopic dermatitis and degree of nickel patch test reactivity. Methods One thousand two hundred and six young adults from the cohort were asked to complete a questionnaire and participate in a clinical examination including patch testing with TRUE Test® including a nickel dilution series. Results The questionnaire was answered by 899 (74.6%), and 442 (36.7%) had patch tests performed. The point prevalence of nickel allergy was 11.8% (clinical relevance 80.8%). The 15-year incidence rate was 6.7%. Most new sensitizations were clinically relevant with strong reactions, and many participants reacted to low concentrations. Only a few positive reactions were lost. Nickel allergy was more common among women with childhood atopic dermatitis, whereas no association with ear piercing was found. However, there was a significantly higher prevalence of nickel allergy among women ear pierced before implementation of the nickel regulation in Denmark. Conclusion This follow-up study in young adults 15 years after leaving primary school showed a high prevalence and a high incidence rate of nickel allergy, despite the nickel regulation. Most reactions from childhood could be reproduced and were clinically relevant. In women, childhood atopic dermatitis was associated with nickel allergy in adulthood, whereas only ear piercing before the Danish nickel regulation was associated with adult nickel allergy.