Case summary This case report describes two cats that had subcutaneous ureteral bypass (SUB) systems implanted and subsequently developed duodenal perforations and septic peritonitis associated with the Dacron cuff of the nephrostomy tube. One cat recovered following surgical explantation of the SUB system with intestinal resection and anastomosis of the perforated small intestine, and – at the time of writing – is still alive. The other cat was humanely euthanased intraoperatively at the owner’s request owing to its perceived prognosis. Relevance and novel information To our knowledge this is the first time this complication has been reported following SUB device placement.