Aggregation of cases of bronchial asthma in adult twin pairs was studied in the nationwide Finnish twin cohort consisting of 13,888 adult monozygotic (MZ) and same-sex dizygotic (DZ) pairs. Cases of asthma were ascertained by record-linkage from three sources of data: death certificates from the Central Statistical Office; hospital discharges from the nationwide hospital registry; and the nationwide registry for fully reimbursed medications of the Social Insurance Institution. A diagnosis of bronchial asthma in one or more of these registers was obtained for 525 (236 men and 289 women) out of 27,776 twin individuals. Among MZ pairs, ten concordant and 138 discordant pairs were identified, while among DZ pairs, 12 concordant and 343 discordant pairs were found. In the whole sample the observed-expected ratio for the number of pairs concordant for bronchial asthma was 4.30 (95 percent confidence interval [CI], 2.06 to 7.90) in MZ pairs and 2.61 (95 percent CI, 1.35 to 4.56) in DZ pairs. Using a multifactorial threshold model, we estimated the correlation in liability to asthma as 0.425 (SE = 0.07) in MZ twins and 0.247 (SE = 0.06) in DZ twins, and we obtained a heritability estimate of 35.6 percent. The data also showed a gender difference in asthma heredity, which may, however, be due to chance events affecting the distribution of concordant pairs.