The Danish Twin Register represents a population-based twin sample where the twins enter the Register independently of disease. All female twins born between 1906-30 and available in the Register in January 1978 were sent a questionnaire concerning possible nickel sensitivity. Among 746 pairs living in the eastern part of Denmark, 129 twins from 115 pairs had a possible nickel allergy. Through a subsequent personal visit and, in most cases, patch testing, 34 monozygotic probands from 30 pairs and 45 dizygotic probands from 41 pairs were considered to have a verified nickel sensitivity and fulfilled the restriction criteria for the present study. The prevalence of present or previous hand eczema in both the monozygotic and the dizygotic probands was 41% (95% confidence limits: 30-52%). In 15 of the 32 with hand eczema, this was in the form of a relapsing pompholyx. Analysis of the monozygotic pairs showed that the risk of developing hand eczema in the co-twins seemed independent of whether the proband had nickel allergy and hand eczema or nickel allergy alone. Furthermore, it was found that the number of affected co-twins was comparable with the background population. Thus the association between nickel allergy and hand eczema is probably not due to a common genetic predisposition. Environmental factors seem decisive.