Eleven of the 35 patients with mediastinal diseases were diagnosed as with sarcoidosis, 6 with tuberculosis, 5 with lymphoma, 11 with nodal metastasis (8 had their modes from the lung and the primary lesions of the other 3 failed to be identified), and 2 with lung cancer with reactive hyperplasia lymph node. The SUV of the delayed images of the 16 malignant lesions was 6.48 (3.0-11.2), higher than that of the early images [6.17 (3.2-9.8)] with a DeltaSUV of 0.31 (-0.4-1.4). The value of SUV of delayed images of the 19 benign lesions was 4.99 (2.2-9.3), lower than that of early images [5.11 (2.9-8.3)] with a DeltaSUV of -0.12 (-0.9-1.0). The DeltaSUV of the benign lesions was significantly lower than that of the malignant lesions (F = 1.939, P = 0.04). The accuracy rates of diagnosis of mediastinal masses of CT, first-time PET-CT, dual time point PET-CT, and videomediastinoscopy were 54.3% (19/35), 74.3% (26/35), 82.9% (29/35), and 100% (35/35) respectively. Conclusion With a high diagnostic yield, videomediastinoscopy remains the gold standard in differentiation of malignant and benign lesions located in the middle mediastinum. Dual time point PET-CT may improve the accuracy.