We found that a cohort of patients with lung cancer first treated in 1977 had higher six-month survival rates for the total group and for subgroups in each of the three main TNM stages (tumor, nodes, and metastases) than a cohort treated between 1953 and 1964 at the same institutions. The more recent cohort, however, had undergone many new diagnostic imaging procedures. According to the "old" diagnostic data for both cohorts, the recent cohort had a prognostically favorable "zero-time shift." In addition, by demonstrating metastases that had formerly been silent and unidentified, the new technological data resulted in a stage migration. Many patients who previously would have been classified in a "good" stage were assigned to a "bad" stage. Because the prognosis of those who migrated, although worse than that for other members of the good-stage group, was better than that for other members of the bad-stage group, survival rates rose in each group without any change in individual outcomes. When classified according to symptom stages that would be unaltered by changes in diagnostic techniques, the two cohorts had similar survival rates.