What is this summary about? This is a summary of the 1-year results of a clinical research study known as CheckMate 649 published in The Lancet in June 2021. The 2-year results on the participants' health and overall quality of life from the same study are in a second publication in Nature in March 2022. Until recently, chemotherapy was the only first treatment option for people with advanced or metastatic gastroesophageal adenocarcinoma who had not been treated before. Patients receiving chemotherapy lived on average for less than 1 year. Nivolumab is an immunotherapy that works by activating a person's immune system to fight back against cancer cells. The goal of CheckMate 649 was to find out if the combination of nivolumab and chemotherapy would help patients with advanced or metastatic gastroesophageal adenocarcinoma live longer and without their cancer getting worse. What were the results? Results from the final analysis are reported here. Of 1581 people who took part in the study, 789 received nivolumab and chemotherapy and 792 received chemotherapy. Researchers found that, on average, participants who received nivolumab and chemotherapy lived longer overall than those who received chemotherapy alone. The length of time participants lived without their cancer getting worse was also longer on average with nivolumab and chemotherapy than chemotherapy treatment alone. However, more participants in the nivolumab and chemotherapy group had side effects than those in the chemotherapy group. The three most common side effects in both types of treatment were nausea (urge to vomit), diarrhea and peripheral neuropathy. Participants who received nivolumab and chemotherapy had a lower risk of their cancer symptoms worsening and reported that they were ‘less bothered’ from side effects of treatment than those receiving chemotherapy alone. What do the results mean? The nivolumab and chemotherapy combination is considered a new standard treatment option and is approved in several countries as a treatment for adults who have not been treated before for their advanced or metastatic gastroesophageal cancer based on results from CheckMate 649. Clinical Trial Registration: NCT02872116 ( ClinicalTrials.gov )