OBJECTIVE Thyroid transcription factor 1 (TTF1) is a transcription factor that regulates the expression of multiple genes involved in lung development. It is preferentially expressed in adenocarcinomas of the lung and has been investigated as a potential prognostic parameter in patients with lung cancer, with conflicting results. We quantitatively assessed TTF1 protein expression in two large and independent data sets to investigate the impact of TTF1 nuclear expression on patient survival. METHODS Automated quantitative analysis, a fluorescent-based method for analysis of in situ protein expression, was used to assess a series of cell lines to find the threshold of detection of TTF1 expression. Then two independent cohorts (176 and 237 cases, respectively) were measured by the same technique, and TTF1 expression was correlated with survival. RESULTS Tumors expressed TTF1 in 45% and 58% of the cases in each cohort. TTF1 was consistently expressed in adenocarcinomas (n = 61 and 73; Spearman rho = 0.313 and 0.4 for the first and second set, respectively; P < .0001) independent of their differentiation and stage. Survival analysis showed that patients with stage I adenocarcinoma with TTF1 expression had a longer median overall survival than those without expression (n = 43, 44.3 v 26.2 months, P = .05 for the first cohort; n = 87; 49.7 v 38.5 months, P = .03 for the second cohort) Multivariate analysis revealed an independent lower risk of death for patients with stage I adenocarcinoma with TTF1-expressing tumors (hazard ratio = 0.479, 95% CI, 0.235 to 0.977; P = .043). CONCLUSIONS TTF1 expression defines a subgroup of patients with a favorable outcome and may be useful for prognostic stratification of patients with stage I lung adenocarcinoma.