8125 Background: Accurate and practical assessment of QL in trials and patient (pt) management are important goals. It should be sensitive to detect relevant changes but not produce excessive burden for pts or staff. This analysis provides a rational basis for the frequency of repeating QL assessment. Methods: Pts were randomized to pemetrexed (P)+ C vs C only (all given q 3 wks). Results demonstrated survival (p=0.02), tumor response (p<0.001, Vogelzang JCO 2003) and QL advantages (p=0.01, Gralla ASCO 2003) for P+C. Pts were prospectively evaluated with the LCSS-Meso, a validated QL instrument, at baseline and weekly while on study. 90% of expected assessments were completed. This analysis evaluates 3 outcomes: total score (mean of all 8 items), average symptom burden index (mean of 5 symptoms), and global QL. Results from weekly assessment for each outcome and for each study arm (P+C or C) were considered as an idealized standard. Area (score × time) was calculated for each interval up to 18 wks; areas were then summed. Goal was to determine whether less frequent assessment would retain similar results to weekly evaluation with lower 95% CI still exceeding 85% (minimal adequacy recommendation, Simon & Wittes Cancer Treat Rep 1985). Results: Results indicate that each assessment period retains >90% of weekly scoring; however, only every 3 wk evaluation exceeds the minimally acceptable CI level. Only 3 wk assessment showed significant between arm differences similar to the weekly interval. Conclusions: Every 3 wk evaluation of QL provides results similar to more burdensome wkly assessment in pts with MPM. A prior analysis in a NSCLC sample yielded the same recommendation (Gralla ASCO 1999), suggesting that every 3 wk evaluation is appropriate in many malignancies with similar clinical characteristics. Author Disclosure Employment or Leadership Consultant or Advisory Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Eli Lilly & Co. Eli Lilly & Co. Eli Lilly & Co. Eli Lilly & Co. Eli Lilly & Co.