Purpose: Fibro-osseous lesions of the jaws are benign lesions that replace bone, generally during childhood or adolescence. Mutation of á subunit of signal-transducing G proteins (Gsá) has been implicated in fibrous dysplasia and the Albright-McCune syndrome. In this study, we examined the Gsá mutation in 24 fibro-osseous jaw lesions. Materials and Methods: The formalin-fixed paraffin embedded samples consisted of eight cases of fibrous dysplasia, 10 cases of cemento-ossifying fibroma, and six cases of focal cemento-osseous dysplasia were selected. Five-micrometer sections of the blocks were prepared and the DNA was then extracted. Subsequently, polymerase chain reaction (nested - PCR) and single-strand conformational polymorphism (SSCP) were carried out to detect the mutations. Results: This mutation was detected in six fibrous dysplasia, nine cemento-ossifying fibroma and four focal cemento-osseous dysplasia samples studied. Statistical analysis of the data (chi-square test) showed no significant difference between these lesions (p=0.0506). Conclusions: This research showed that a unique mechanism must be responsible for the creation of fibro-osseous lesions, and the possibility of other unknown mutations may be responsible for the differences of these lesions. To confirm our evaluations, DNA sequencing of the fragments is being done.