Objective.99mTc-ethylenedicysteine-glucosamine (99mTc-EC-G) was developed as a potential alternative to 18F-FDG for cancer imaging. A Phase 2 study was conducted to compare 18F-FDG PET/CT and 99mTc-EC-G SPECT/CT in the detection and staging of patients with non-small cell lung cancer (NSCLC). This study was aimed to demonstrate that 99mTc-EC-G SPECT/CT was not inferior to 18F-FDG PET/CT in patients with confirmed NSCLC. Methods. Seventeen patients with biopsy proven NSCLC were imaged with 99mTc-EC-G and 18F-FDG to detect and stage their cancers. Imaging with PET/CT began 45-60 minutes after injection of 18F-FDG. Imaging with 99mTc-EC-G began at two hours after injection (for 5 patients) or three hours (for 12 patients). SPECT/CT imaging devices from the three major vendors of SPECT/CT systems were used at 6 participating study sites. The image sets were blinded to all clinical information and interpreted by independent PET and SPECT expert readers at a central independent core laboratory. Results. 100% concordance between 99mTc-EC-G and 18F-FDG for primary lesion detection, lesion location and size, and confidence that the biopsied lesion was malignant. There was 70% agreement between 99mTc-EC-G and 18F-FDG for metastatic lesion detection, location and size, and confidence that the suspicious lesions were malignant. Conclusions. Evaluation of primary and suspicious metastatic lesions detected by 99mTc-EC-G and 18F-FDG on 17 patients resulted in excellent agreement for detection of primary and metastatic lesions. The study results indicated that 99mTc-EC-G SPECT/CT has the potential to be a clinically viable alternative to 18F-FDG PET/CT and 99mTc-EC-G is not inferior to 18F-FDG PET/CT.