Background: While immune-checkpoint inhibitors (ICIs) have transformed the field of oncology for advanced-stage cancers, they can lead to serious immune toxicities. Several systematic reviews have evaluated the risk of immune-related adverse events (irAEs); however, most have focused on published articles without evaluating trial registries. The objective of this methodological review was to compare the quality of reporting of safety information and in particular, serious irAEs (irSAEs), in both publications and ClinicalTrials.gov for all current FDA-approved ICIs. Methods: PubMed was searched to retrieve all published phase III randomized controlled trials (RCTs) evaluating ICIs. For each eligible trial, we searched for corresponding registration on ClinicalTrials.gov and extracted relevant safety data from both the publication and results posted on registry. We then compared the quality of reporting and the value of safety data between both sources. Results: Of 42 eligible published trials, 34 had results posted on ClinicalTrials.gov . Considerable variability was noted in the reporting of safety in both sources. SAEs were reported for all trial results in ClinicalTrials.gov compared to 23.5% of publications. An overall incidence for irAEs and irSAEs was reported in 58.8 and 8.8% of publications respectively, compared to 11.8 and 5.9% in registry results. Comparing the value of specific irSAEs was not possible between the two sources in 32/34 trials either due to different reporting formats (61.8%) or data not being reported in one or both sources (32.4%). From the 2 studies with compatible irSAE format, only 1 had matching data in both sources. Conclusions: The reporting of irAEs / irSAEs varies considerably in publications and registries, which outlines the importance of standardizing the terminologies and methodologies for reporting safety information relevant to ICIs.