The diagnosis of osteoid osteoma is usually not difficult. In typical forms of the disease, with diaphyseal localization, the clinical course and radiographic findings are so typical that diagnosis may be considered certain even when there is no histological confirmation. In some localizations, nonetheless, such as in the cancellous or short bones, or when the lesion involves a joint, diagnosis may be difficult, uncertain and late. Between January 1991 and April 1997 a total of 91 patients who had been diagnosed as having osteoid osteoma were submitted to computed tomography. Of these patients, there were 34 females, and 57 males, and age ranged from a minimum of 2 to a maximum of 69 years. Localizations are reported in Table I. For the purposes of our study, several cases with atypical features have been selected from these. We found it interesting to emphasize the importance of a correct diagnostic procedure with the purpose of accurately identifying the lesion.