Eight patients with a pyogenic infection of the sacroiliac joint are compared to 200 published cases. In all our patients the disease began with fever and immobilizing low back and buttock pain. All had bacterial growth in the blood cultures. Five out of 6 patients did not show inflammatory signs in the initial plain roentgenogram. Tc-99m scan was initially positive in 4 out of 6 patients. In 2 patients only the second scan, at 13 and 15 days respectively, was positive. All but one patient had a 4-6 week course of intravenous antibiotics. Three patients underwent surgery for abscesses or intraarticular sequestra. One patient with a small psoas abscess had only medical treatment under CT monitoring. All the patients recovered. From our observations and the literature we conclude that pyogenic sacroiliitis is often not recognized initially. Wrong diagnoses such as sepsis of unknown origin, appendicitis, discal hernia etc. can be avoided if pyogenic sacroiliitis is sought in a systematic fashion. The clinical diagnosis can be confirmed by bone scan, to be repeated at a later stage of disease (i.e. two weeks after onset) if the first examination is inconclusive.