Background: In the Phase 3 POSEIDON study in 1L mNSCLC, adding T to D+CT resulted in statistically significant improvements in PFS and OS vs CT. No new safety signals were identified and treatment discontinuations due to treatment-related AEs (TRAEs) were similar for the T+D+CT and D+CT arms (15.5% and 14.1%). Here we present details of AEs and their management. Methods: 1013 pts with EGFR/ALK wild-type mNSCLC were randomized 1:1:1 to 1L T+D+CT, D+CT or CT. Safety was assessed in all treated pts. Results: 330, 334 and 333 pts received T+D+CT, D+CT and CT; 78%, 82% and 74% received at least 4 cycles of platinum-based CT. The most common grade 3/4 TRAEs were hematologic (anemia in 17%, 15% and 20% of pts in the T+D+CT, D+CT and CT arms and neutropenia in 16%, 13% and 12%) and most were managed using standard approaches per local practice; 22%, 18% and 16% of pts received colony stimulating factors and 22%, 21% and 26% received blood transfusions. All grade immune-mediated AEs (imAEs) occurred in 34%, 19% and 5% of pts in the T+D+CT, D+CT and CT arms; a higher incidence of diarrhea/colitis, dermatitis/rash and endocrinopathies was seen with the addition of T to D+CT (Table). Grade 3/4 imAEs occurred in 10%, 7% and 2% of pts in the T+D+CT, D+CT and CT arms, and serious imAEs in 10%, 6% and 1%; imAEs led to discontinuation of any study treatment in 6%, 4% and 0.6%, and led to death in 0.6%, 0.3% and 0%. Most imAEs were low grade and manageable with systemic corticosteroids (received by 26%, 13% and 4% of pts in the T+D+CT, D+CT and CT arms) or endocrine therapy (12%, 8% and 1%). Median time from first dose to onset of imAEs (TTO) was generally > 60 days and the majority of non-endocrine imAEs resolved (Table). Conclusions: In POSEIDON, the safety profile of all regimens was manageable per standard guidelines and in line with the known profiles of D, T+D and CT; the most common grade 3/4 TRAEs were those typically associated with CT. As expected, more imAEs occurred with T+D+CT than D+CT, but the incidence of grade 3 or 4 imAEs, imAE-related deaths and treatment discontinuations due to imAEs was generally similar in the IO arms. T+D did not compromise the ability to administer planned CT. Clinical trial information: NCT03164616.