Background: Standard front-line chemotherapy for advanced MPM is (Pem and a platinum; optimal treatment duration is unknown. We performed a randomized phase 2 trial (NCT01085630) to determine if continuation of single-agent Pem after 4-6 cycles of Pem-platinum would improve progression-free survival (PFS). Methods: Eligible pts had histologically confirmed unresectable MPM, and performance status (PS) 0-1. Pts with at least stable disease following 4-6 cycles of Pem-platinum were stratified by first-line regimen (cis- or carboplatin) and histology (epithelioid versus other) and randomized 1:1 to Obs or continuation of Pem until progression. The primary endpoint was PFS. We assumed that Obs produced a median (m) PFS of 3 months (mo) and Pem would yield a 100% improvement in mPFS to 6 mo; 60 eligible pts (30 per arm) were to be randomized. Results: 72 pts from 30 sites registered 12/10-6/16. The study closed early due to slow accrual once 53 pts were randomized; 49 eligible pts (22 Obs, 27 Pem) are included in the efficacy analysis. Pt characteristics (Obs/Pem): age: median (range) 70 (39-85)/70 (52-87); male 68%/78%; PS 0 27%/33%; epithelioid histology 77%/70%; first-line cisplatin 27%/26%. A median of 4 cycles of Pem (range 1-33) was delivered; 22% of pts required dose modification. mPFS was 3 mo on Obs and 3.4 mo on Pem (hazard ratio (HR) 0.99; 95% CI: 0.51-1.90; p=0.9733). Median overall survival (mOS) was 11.8 mo for Obs, and 16.3 mo for Pem (HR 0.86; 95% CI 0.44-1.71; p=0.6737). Toxicities ≥ grade 3 on Pem included anemia 8%, lymphopenia 8%, neutropenia 4%, and fatigue 4%; there were no grade 5 toxicities. A higher baseline level of serum mesothelin related peptide (SMRP) was associated with worse PFS (HR 1.861, p=0.049). Baseline osteopontin did not significantly affect PFS (p=0.3630). Conclusions: Although it was well tolerated, maintenance Pem following initial Pem/platinum doublet chemotherapy does not improve PFS in MPM patients. High baseline SMRP was associated with shorter PFS. Support: U10CA180821, U10CA180882; https://acknowledgments.alliancefound.org. Clinical trial information: NCT01085630