Seventy patients with newly diagnosed, pathologically proven inoperable non-small cell lung cancer (NSCLC) had planar Ga-67, Tl-201, chest x-ray, and chest CT imaging performed. Tumor/Normal tissue background (T/B) ratio was calculated for 62 Ga-67 and 55 Tl-201 scintigraphy studies and comparisons were made between Ga-67 and Tl-201 imaging results regarding T/B ratios, site of lesion, and histologic type. The impact of the images on the initial knowledge of the extent of the tumor and on the radiotherapy (RT) planning was evaluated for each patient. For primary lesions, Ga-67 imaging results were positive in 94% (66 of 70 patients) versus 71% (50 of 70 patients) for Tl-201 scans (P < 0.005) and the T/B ratio was > 1.5 in 74% (46 of 62 patients) for Ga-67 versus 36% (20 of 55 patients) for Tl-201 (P < 0.0001). For centrally located lesions, sensitivity for Ga-67 was 100% (53 of 53 patients) versus 74% (39 of 53 patients) for Tl-201 (P < 0.0005) and the T/B ratio > 1.5 in 84% (38 of 45 patients) for Ga-67 versus 38% (15 of 40 patients) for Tl-201 (P > 0.001). For peripheral lesions, sensitivity of Ga-67 was 76% (13 of 17 patients) versus 64% (11 of 17) for Tl-201 (P > 0.05) and the T/B ratio was > 1.5 in 47% (8 of 17 patients) for Ga-67 versus 33% (5 of 15 patients) for Tl-201 (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)