To assess the impact of disease and treatment on patients with advanced non-small cell lung cancer (NSCLC), we set out to determine a clinically meaningful change (CMC) on the Lung Cancer Subscale (LCS) and the Trial Outcome Index (TOI) of the Functional Assessment of Cancer Therapy-Lung (FACT-L) questionnaire. We used data from Eastern Cooperative Oncology Group study 5592 (E5592), a randomized trial comparing three chemotherapeutic regimens in 599 advanced NSCLC patients. Patients completed the FACT-L at baseline (pretreatment), 6 weeks, 12 weeks, and 6 months. Comparing across baseline performance status (0 vs. 1), prior weight loss (<5% vs. > or = 5%), and primary disease symptoms (< or = 1 vs. >1), LCS and TOI score differences ranged from 2.4 to 3.6 and 6.5 to 9.2, respectively (all Ps <.001). Mean improvement in LCS score from baseline to 12 weeks was 2.4 points in patients who had responded to treatment versus 0.0 points in patients who had progressive disease. Twelve-week LCS change scores for patients progressing early were 3.1 points worse than those of patients progressing later (mean = -1.2 vs.1.9, respectively). Similarly, the average TOI change score from baseline to 12 weeks was -6.1 for patients who had progressive disease versus -0.8 points for patients who had responded to treatment. Twelve-week TOI change scores for patients progressing early (mean = -8.1) were 5.7 points worse than those of patients progressing later (mean = -8.1 vs. -2.4, respectively). Analyses assuming nonrandom missing data resulted in slightly larger differences. Clinically relevant change scores were estimated as two to three points for the LCS and five to seven points for the TOI, setting upper limits for minimal CMCs. These values were comparable to suggested distribution-based criteria of a minimally important difference. These results support use of a two to three point change in the LCS and five to six point change on the TOI of the FACT-L as a CMC, and offer practical direction for inclusion of important patient-based endpoints in lung cancer clinical trials.