Peritonitis is a potentially life-threatening disease in horses. With no published work from Sweden, the objective of the study was to describe the occurrence and demographics of horses with peritonitis and to evaluate the outcome of treatment in terms of short-term survival during a ten-year period in a large clinic in Sweden. Data were examined in a retrospective manner in 69 horses diagnosed with and treated for peritonitis at Universitetsdjursjukhuset (UDS) in Uppsala between 2002-2012. Demographic data examined included age, breed, gender, history, duration of illness before arriving at UDS, reason for seeking veterinary care, initial clinical findings (general state of health, heart rate, respiratory rate, mucous membrane appearance, rectal temperature, abdominal sounds, rectal examination, nasogastric tube results), abdominocentesis results (abdominal fluid analysis including visual inspection, leukocytes and protein, cytology, bacterial culture and sensitivity pattern), complete blood count (CBC), Serum amyloid-A (SAA) upon presentation and a follow up, plasma fibrinogen, plasma protein and albumin, treatments, length of hospitalisation and outcome. All medical records with the diagnosis of peritonitis were extracted from the medical records system Trofast. All records with any other diagnosis code referring to trauma such as rectal tear or ruptured uterus, recent abdominal surgery, external trauma or rupture in the gastrointestinal tract were excluded. To be included in the study, the peritoneal fluid should contain more than 20.000 cells/?L, have a peritoneal protein value of >30 g/L; or have a significantly changed peritoneal fluid sample (orange with increased turbidity or worse) in cases where no data on cells or protein were available. The horses were divided into two groups; one where the peritonitis was deemed to have an idiopathic aetiology (primary peritonitis) and the other group where the peritonitis had a possible aetiology such as intestinal parasites, impaction etc.