The genetic predisposition of nickel allergy was evaluated in 1,546 Danish female twins born between 1906 and 1930. The twins were investigated primarily via a mailed questionnaire and were asked if they noticed redness, itching, or dermatitis when they came in contact with metal clasps or costume jewelry. The material was divided into two equal parts corresponding to the western and eastern parts of Denmark. The majority of the possible positives in the eastern part were examined and patch tested.