The FACT-L (version 3) is a 44-item self-report instrument which measures multidimensional quality of life. Available in eight languages, it is currently being used in several Phase II and III lung cancer clinical trials. Reliability and validity of the 33-item version 2 of the FACT-General (FACT-G) have previously been published. This paper reports further validation data on the FACT-G with a subsample of lung cancer patients from the original publication and, more importantly, presents data on the Lung Cancer Subscale (LCS). The nine LCS questions were administered along with the FACT-G to 116 patients with lung cancer. Internal consistency (coefficient alpha) was improved from 0.53 to 0.68 by dropping two questions which were uncorrelated with the others. A subset of 41 patients was tested again at 2 months to evaluate sensitivity to change in performance status rating (PSR) and to obtain estimates of a clinically meaningful change score for the FACT-G and the 7-item LCS. Using a linear test for trend, sensitivity to change in performance status rating (PSR) was obtained with the Total score (P = 0.03), the Physical Well Being (PWB) subscale (P = 0.02), the Functional Well Being (FWB) subscale (P = 0.05), and the LCS (P = 0.03). A 21-item Trial Outcome Index (TOI), combining scores on PWB, FWB and LCS, was highly reliable (coefficient a = 0.89) and sensitive to change in PSR F(1,38) = 4.84 (P = 0.01). This TOI is probably the most relevant and precise indicator of patient-reported quality of life available for lung cancer patients who complete the FACT-L while participating in an oncology clinical trial. The FACT-L may also be of benefit in evaluating quality of life in patients with lung diseases other than cancer.