This study enrolled 143 asymptomatic patients with lung cancer detected by mass screening during an 8-year period (January 1, 1993 to December 31, 2000) and who had received a lung cancer mass screening roentgenogram one year before the disease was found. There was no difference between the 5-year survival rates in patients with one-year delayed detection of lung cancer (n=62) and in patients without (n=81) (46 vs. 58%, log rank: P=0.1330, Wilcoxon: P=0.1008). However, according to the tumor size on the overlooked chest roentgenogram, the outcome in stage I+II patients with missed tumors >20 mm in dimension (n=20) was worse than those with missed tumors <10 mm (n=24) or those with missed tumors 10-20 mm (n=18) (40 vs. 82 or 81%, log rank: P=0.0047, Wilcoxon: P=0.0010). All missed tumors in the lung field that did not overlap thoracic components were <10 mm in dimension and appeared as patchy ground-glass opacities, and they could not have been recognized if there was no other information that the tumor developed in that location. This might also be related to the lack of mortality effectiveness of previous lung cancer mass screening problem. Although it may be difficult to find the tumors <10 mm on a chest roentgenogram on mass screening, one-year delayed detection of lung cancer < or = 20 mm will not affect the prognosis.