We report a case of spontaneous rupture of a bladder diverticulum. A 65-year-old man presented with urinary retention and serious infection. At clinical examination, the patient had a distended abdomen, tenderness of the left flank and left lumbar region and benign prostatic hyperplasia. Urinary catheterisation yielded bloody and purulent urine. CT scan showed a paravesical collection extending to the left lumbar region. Blood count showed anaemia and leukocytosis. Emergency surgical exploration found a large infected bladder diverticulum with necrosis and perforation of the wall. Treatment consisted of resection of the necrotic tissue and suture of the diverticulum neck. Convalescence was uneventful.