To assess the extent of overdiagnosis bias in lung cancer screening, clinical Stage I lung cancer cases detected by chest radiograph examination, with histologic or cytologic evidence of malignancy and not treated by surgical operation, were followed up for more than 10 years. Of 1297 screen-detected and 1297 symptom-detected cases collected from 20 institutions, 42 screen-detected and 27 symptom-detected cases satisfied the study criteria. In about half of the cases, the patients had no contraindication for surgical treatment, but they refused surgical procedure. All such patients from the screen-detected and symptom-detected groups died within 122 and 67 months, respectively, of diagnosis. Among the screen-detected and symptom-detected cases, 80% and 81%, respectively, of the patients died of lung cancer. The median survival time was 25 and 13 months for those in the screen-detected and symptom-detected groups, respectively. The difference in survival was statistically significant between the two groups, which indicated the effect of lead time and length-biased sampling. Analysis of the causes of death other than lung cancer showed that there was no difference in the observed cumulative rates of deaths of other causes between the two groups, and these figures were almost the same as those expected from the general population. This indicates that overdiagnosis bias would be minimal in screen-detected lung cancer cases detected by chest radiograph examination.