The authors present a case of osteoid osteoma (O.O.) localized in the ischiatic ramus, a very rare (incidence varying from 1.2% to 2.7% in the literature) site for this lesion. Of all the diagnostic tests used, including conventional radiography and Computerized Axial Tomography (CT), the information provided by Magnetic Resonance Imaging (MRI) was particularly valuable. MRI permits recognition of the "nidus" of the osteoid osteoma, which usually emits an intermediate signal on T1-"weighted" images, and is more sensitive than conventional radiography or CT, especially when the marked sclerotic reaction around the tumor renders diagnosis with these last two methods difficult.