Most patients with sarcoidosis develop lung involvement, and its disease activity in the lung parenchyma is often critical for making decisions for treatment. Therefore, evaluating lung involvement accurately is of great concern. The purpose of this study was to investigate usefulness of the retention index calculated by dual-time point fluorine-18-fluorodeoxyglucose positron emission tomography (FDG-PET) for the detection of active sarcoidosis lesions in the lung parenchyma in comparison with other imaging techniques: CT scan and standard uptake values at 60 min (SUV_{60 min}). We retrospectively reviewed medical records and imaging of seventeen sarcoidosis patients with FDG-PET before any treatment. Retention index of FDG uptake was calculated as ([SUV at 120 min]-[SUV_{60 min}])×100/SUV_{60 min}. When we determined the threshold of SUV_{60 min} and the retention index for lung parenchymal involvement of sarcoidosis at 0.663, 5.423%, we found that these parameters were as useful as CT findings.