Because of the frequency with which Irish wolfhounds were observed to have intrahepatic portosystemic shunts, the concentration of ammonia was measured in samples of venous blood taken at seven to eight weeks old from the 1066 Irish wolfhounds born in the Netherlands between 1984 and 1992. The average rate of incidence of the condition (defined as an ammonia concentration exceeding 150 microM) over this period was 2.1 per cent and it increased (P < 0.03) at an average annual rate of 0.34 per cent. The genetic similarity between the individual dogs with portosystemic shunts, defined by their degree of relationship (Rpq), was significantly higher (P < 0.00001) than either the Rpq among the control dogs which had an ammonia concentration below 46 microM or the Rpq between the affected dogs and the controls. Furthermore, the mean ammonia concentration in clinically normal Irish wolfhounds was higher than that in the healthy control dogs of various breeds (73.8 v 33.4 microM). The overrepresentation of portosystemic shunts in Irish wolfhounds, the increasing rate of incidence of the shunts, and the familial distribution of the animals with the condition are evidence for a hereditary basis for the disorder in this population.