Flow cytometry analysis has been used to measure the S phase in paraffin embedded osteosarcoma tissues of 22 children and adolescents who were surgically treated. The aim of this study was the assessment of S phase cell cycle in relation to survival, localization of tumour and histopathological grading. Our findings demonstrate the usefulness of flow cytometry determination of S phase cell cycle as a prognostic factor. Higher percentage of S phase cells predict worse prognosis.