The levels of proliferating cell nuclear antigen (PCNA) are almost negligible in long-term quiescent cells and increase dramatically during the cell cycle. Recently, the monoclonal antibodies to PCNA have been used to demonstrate the proliferative component of paraffin-embedded tumor tissues. PCNA has been shown to be available as a simple histologic marker of proliferative activity, and the PCNA labeling index has been correlated with the prognosis of several malignant neoplasms. Formalin-fixed, paraffin-embedded tissue specimens of 18 primary pediatric osteosarcomas were immunostained by using an anti-PCNA monoclonal antibody (Dako PCNA PC 10). The relationship between the PCNA index, clinicopathologic features and survival were assessed retrospectively. The mean PCNA index for the whole series was 42%. There was no correlation between the PCNA index and age, sex, histopathologic necrosis rate after preoperative chemotherapy, metastasis at diagnosis, relapse or progressive disease and survival rates. According to the results with the limited number of patients in this series, the PCNA labeling index does not seem to be a prognostic factor in osteosarcoma.