The aims of the studies was to evaluate the short and long term effects on the development of parastomal hernia and stoma complications of a prophylactic prosthetic mesh placed in a sublay position at the index operation.  Also the purpose was to validate a definition of parastomal hernia at clinical examination and a method and a definition of parastomal hernia at CT-scan. In the first two studies 27 patients were randomized to a conventional stoma or to a stoma with the addition of a partly absorbable low weight large pore mesh in sublay position. Patients were examined after one and five years.  After five years the rate of parastomal hernia was 80% with a conventional stoma and 14% with the addition of a mesh.  A prophylactic mesh did not increase the rate of complications. In the third study a prophylactic mesh was intended at stoma formation in 93 consecutive patients in routine surgery. In 75 patients provided with a mesh the rate of parastomal hernia after one year was 13%. Complication rates were not increased in 19 severely contaminated wounds. In the fourth study 27 patients with ostomies were examined by tree surgeons and parastomal hernia was defined as any protrusion in the vicinity of the stoma. CT-scans with patients examined in the supine and prone positions were assessed by three radiologists. Herniation was then defined as any intra abdominal content protruding beyond peritoneum or the presence of a hernia sac. Kappa was 0.85 for surgeons and 0.85 for radiologists with CT-scan in the prone position. Kappa was 0.80 for surgeons and radiologists collectively, with CT-scan in the prone position. Four parastomal hernias detected at CT-scan in the prone position could not be detected in the supine position. A parastomal hernia diagnosed at clinical examination was always detected at CT-scan in the prone position. Conclusions: A prophylactic mesh placed in a sublay position at the index operation reduces the rate of parastomal hernia without increasing the rate of complications. Parastomal hernia should at clinical examination be defined as any protrusion in the vicinity of the stoma with the patient straining in the supine and erect positions.  At CT-scan, with the patient examined in the prone position, herniation should be defined as any intra abdominal content protruding beyond peritoneum or the presence of a hernia sac. / Embargo, publiceras 2011-05-01