The cases of seventeen children whose ages ranged from two to eighteen years and who were treated for a disorder of a sacro-iliac joint between 1975 and 1983 were reviewed retrospectively. Thirteen children were acutely ill, with a temperature of more than 38 degrees Celsius, and four had chronic symptoms that had persisted for three weeks to one year. Pain in the hip, thigh, and buttock was the most common symptom. Of the thirteen acutely ill patients, eleven had septic arthritis of a sacro-iliac joint, while one who had ankylosing spondylitis and one who had juvenile rheumatoid arthritis had acutely painful arthritis of a sacro-iliac joint. Of the four patients who had chronic symptoms, two had septic arthritis of a sacro-iliac joint; one, ankylosing spondylitis with sacro-iliac involvement; and one, eosinophilic granuloma of the ilium. Thus, thirteen patients had septic arthritis of a sacro-iliac joint and four had some other disorder. For the seventeen children who had acute or chronic symptoms, at admission the white blood-cell count ranged from 3,500 to 26,200 per cubic millimeter (average, 11,100 per cubic millimeter) and the sedimentation rate, as determined by the Westergren technique, ranged from twenty-two to sixty-five millimeters per hour (average, fifty millimeters per hour). Twelve of the plain radiographs of the seventeen patients were negative. The initial bone scans of all seventeen patients were positive in eleven and negative in six. Of these six, five had septic arthritis and one, juvenile rheumatoid arthritis. A computed tomographic scan was performed in four patients and was positive in all of them: three had septic arthritis and one had ankylosing spondylitis. Organisms were cultured successfully from blood, from material aspirated from the sacro-iliac joint, or from stool of all thirteen patients who had sepsis. The thirteen infections responded well to appropriate antibiotics, which were administered intravenously to seven patients and first intravenously and then orally to six.