Osteomyelites, bone infections of a hematogenous origin, are rare in the pelvis (2.3%) and are extremely rare in the ischium. Ischiatic osteomyelitis is usually found in children and adolescents, but has rarely been described in adults. The clinical presentation varies and the diagnosis is based on magnetic resonance imaging (MRI). The most frequently isolated germ is the staphylococcus, while Escherichia coli has been found in a few cases. We report a case of osteomyelitis from E. coli in a 46-year-old woman revealed by persistent fever. The point of entry was a septicemia from gastrointestinal origin, related to colon polyps. The clinical picture was also complicated by an antiphospholipid antibody syndrome (superior mesenteric vein and splenomesenteric branch thrombosis). The course was favorable thanks to appropriate antibiotic treatment and surgical debridment of the infection.