To identify large numbers of twins affected by chronic disease as potential subjects for studies of environmental and genetic chronic disease determinants, we advertised for affected twins over the period 1980–91 in newspapers across North America. Responses were received from 17 245 twin pairs in which cases of cancer or other chronic disease had occurred. To assess the representativeness of affected twins identified by advertising, we evaluated the pattern of reporting, compared the cases identified to the number of cases estimated to be prevalent among all North American twins, compared the cases to population-based singleton case series, compared the healthy co-twins to population-based samples of healthy persons, assessed the impact on ascertainment of opinions about disease causation, compared the pattern of prospective to retrospective ascertainment of disease in the originally unaffected co-twins of cases, and compared the results of the prospective ascertainment of disease in co-twins to comparable published estimates. Youth, gender, zygosity, education, and disease concordance were found to be overall determinants of ascertainment. Disease-discordant DZ twins appeared to be modestly underascertained. While somewhat better educated, both concordant and discordant pairs were judged to be reasonably representative of affected non-Hispanic white North American twin pairs of comparable status, ie of comparable age, sex, race, and zygosity. If interpreted with caution, the concordance patterns of such twins can be used to generate genetic hypotheses, but should not be the basis of definitive heritability analyses. We conclude that advertising offers a method of identifying pairs of twins that can serve as subjects for studies designed to identify disease determinants. Twin Research (2000) 3, 33–42.