The percutaneous transhepatic portal approach is the most commonly used technique for islet transplantation, largely owing to its safety and minimally invasive characteristic. Bleeding complications after islet transplantation are rare and include portal vein thrombosis and subcapsular liver hematoma. We report a massive hemothorax after portal vein catheterization in a patient with brittle type 1 diabetes undergoing hepatic islet embolization. The patient was under long-term aspirin therapy because of vascular complications and received heparin in low doses to prevent the instant blood-mediated inflammatory reaction and reduce the risk of portal vein thrombosis. The present case illustrates the particular risk of bleeding complications in patients with brittle type 1 diabetes, who represent a frail population. This uncommon adverse event highlights the importance of close monitoring in the first days following islet transplantation.