Five patients with septic sacroiliitis, 3 unilateral and 2 bilateral, are described. The illness was acute, with high fever, low back or buttock pain, severe sacroiliac tenderness and elevated erythrocyte sedimentation rate. Staphylococcus aureus was isolated from the blood in 3 cases and Salmonella typhi in 1. Diagnosis was confirmed by scintigraphy in 4 cases. Roentgenographic changes developed in 2 patients. Response to antimicrobial therapy was favourable. Awareness of this condition is mandatory for prompt diagnosis and correct therapy. Sacroiliac tenderness with increased radionuclide uptake, high fever and positive blood culture are the primary diagnostic criteria.