Histopathological changes are reported in the livers of cats with congenital portosystemic shunts (CPSS) before and after surgical attenuation. Medical records, portovenograms and liver biopsies from cats treated surgically for CPSS were reviewed. Biopsies were graded for histopathological features characteristic of CPSS. Of 40 cats with CPSS included in the study, all had portal vein hypoplasia and arteriolar hyperplasia at initial surgery, 20 (50 per cent) had hepatocyte swelling with microvesicular vacuolar change, 17 (42.5 per cent) had fibrosis, 12 (30 per cent) had hepatocyte swelling with macrovesicular vacuolar change, 8 (20 per cent) had biliary hyperplasia and 2 (5 per cent) had haemosiderin within Küpffer cells. Cats with macrovesicular vacuolar change were significantly older than cats without (P=0.001), with median ages of 18.5 months and 8.5 months, respectively. Twenty-five cats had partial attenuation of the CPSS at initial surgery, and 16 of these had follow-up biopsy samples. There were no significant differences in the histopathological features of biopsies before and after partial attenuation. From first to second surgery, there was a significant improvement in intrahepatic vasculature on portovenography both before (P=0.001) and after (P=0.039) temporary complete attenuation. Following partial CPSS attenuation, there was no significant change in histopathological features despite an improvement in intrahepatic vasculature on portovenography.