In Norway specified information of marriages between second cousins and closer relations has been recorded in the marriage certificates since January 1889. In addition, information was collected in the 1891 census and in the Medical Registration of Births (established in 1967). A ratio of second- to first-cousin marriages equal to or greater than 2 has been taken as some evidence indicating reliability of the source of data. Using this criterion, the information in the Medical Registration of Births seems most reliable, and that in the 1891 census possibly of similar accuracy. On the other hand, from 1890 the marriage certificates are unreliable with regard to the proportion of second-cousin marriages (ratio less than 1), and with regard to the proportion of first-cousin marriages from 1919. The present inbreeding in Norway is low (alpha4 = 0-000242), approximately one-eighth of the estimated level according to the 1891 census. The greatest reduction in inbreeding probably occurred after 1920.