The effect of migration on pairwise concordance for disease was assessed in 11,154 twin pairs of the Finnish Twin Cohort Study by comparing the pairs living in the same province to the pairs which members were living in different provinces of Finland. The cumulative incidence of psychosis and hypertension for the years 1972-1985 were analyzed. The cumulative concordance of psychosis for those MZ twin pairs living in the same province were higher than for those MZ pairs living further apart. Similar findings were found among DZ pairs for psychosis. The cumulative concordance of hypertension was only slightly higher among those MZ and DZ pairs living in the same province compared with pairs living in different provinces. These results indicate an overestimation of concordance of psychosis caused by selective migration. This bias in twin studies is likely to influence heritability estimates in a sample of limited geographical area.