The clinical features, macroscopic and radiologic findings and typical histologic alterations in three cases of fibrous dysplasia of ribs are described. In one case the lesion presented as a tumor-like growth of a size unprecedented in previously reported cases. In differentiating this condition from ossifying fibroma, the pattern of distribution of newly formed bone to fibrous connective tissue is shown to be a valuable criterion. In fibrous dysplasia this ratio is constant for any given area, but varies considerably in ossifying fibroma.