Portal hypertension caused by primary hypoplasia of the portal vein was diagnosed in 42 dogs. The portal hypertension was manifested by the presence of multiple portosystemic collateral vessels. The main clinical signs were retarded growth or weight loss, apathy, intermittent diarrhoea and vomiting, anorexia, abdominal distension and polydipsia. Major findings at physical examination were ascites in 23 dogs and neurological signs in 16 dogs. The dogs had increased activities of liver enzymes in plasma and increased fasting levels of total bile acids and ammonia; in many of the dogs the packed red cell volume, total serum protein and albumin were low. Gross inspection of the portal vein revealed a patent but underdeveloped extrahepatic vein in 13 of the dogs. Microscopic examination of the liver revealed hypoplasia of the intrahepatic portal veins in all the dogs, and this was associated with minor arteriolar proliferation and absence of fibrosis in 12 of them, with moderate to marked arteriolar proliferation often combined with ductular proliferation in 13, and with marked portal fibrosis (formerly described as hepatoportal fibrosis) with a varying number of arteriolar and bile ductular structures in 17 of the dogs. The disease affected mainly young dogs, and was most likely to have been of congenital origin.