Background: Neutrophil-to-lymphocyte (NLR) and platelet-to-lymphocyte (PLR) ratios can indicate poor disease prognosis and are inflammation markers. We investigated the role of NLR and PLR as effective predictive markers of immune-related adverse event (irAE) onset in patients treated with nivolumab. Methods: We retrospectively analysed 73 gastric and renal cancer patients treated with nivolumab at the Hokkaido Cancer Centre from January 2017 to June 2020. NLR and PLR were calculated at the initiation of nivolumab treatment and irAE onset. We identified the risk factors for Grade 3-4 irAE onset using NLR, PLR, sex, cancer type, and age. Overall survival (OS) and progression free survival (PFS) were calculated from the initiation of nivolumab treatment to the date of death or censored at last follow-up. Results: Among the 73 patients included, 17 (18%) had at least one grade3-4 irAE. Multivariable logistic regression analyses revealed that pretreatment NLR<4.3 was significantly associated with a reduced risk for onset of grade3-4 irAEs, whereas rate of NLR change after treatment, ΔNLR>120% was significantly associated with an increased risk. Conclusions: NLR is an effective marker for prognosis and onset of grade 3-4 irAEs.