We report clinical and laboratory findings in 11 patients with septic sacro-iliitis. Fever, pain and immobility were prominent symptoms. Acute tenderness of the sacroiliac joint and elevation of the ESR were present in all. Joint puncture allowed identification of the infectious agent in 10 patients: S aureus 4, S typhi 3, E coli 2 and P mirabilis 1. Good clinical results were obtained after appropriate antibiotic therapy.