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Abstract Background Casirivimab-imdevimab has been developed to neutralize SARS-CoV-2. The global clinical trials in outpatients documented several adverse effects (AE), which mandate caution in Japan where part of patients return home. To investigate post-infusion clinical events and their risk factors, we attempted a retrospective study. Main body Subjects were a consecutive series of inpatients with COVID-19 undergoing an infusion of casirivimab-imdevimab in our institute. The criteria for administration were in accordance with previous clinical trials, e.g., exclusion of patients necessitating oxygen supply. In Japan, however, SARS-CoV-2 vaccinees were eligible. Methods were review of background factors of status, imaging, and laboratory findings for the outcome of post-infusion events such as temperature increase (Temp+), pulse oximetry below 94%, and other events. Also, we documented the drug efficacy. Of a total of 96 patients with a median follow-up of 54 days, one (1.0%) died who alone was an exception demanding oxygen supply. Other 95 patients (99.0%) recovered from fever and hypoxia by Day 4 and later had no worsening of COVID-19. Median increase of body temperature was 1.0 degrees Celsius, which was used for computation of Temp+. Multivariate analysis showed that for Temp+ (n = 47), white blood cell counts more than 4.3 × 103/microliter (Odds Ratio [OR] 2.593, 95% Confidence Interval [CI] 1.060–6.338, P = 0.037) was at risk, whereas 2-time vaccination for SARS-CoV-2 (OR 0.128, 95% CI 0.026–0.636, P = 0.012) was a preventing factor. Likewise for lowered oximetry (n = 21), CT showing bilateral ground glass attenuation (OR 5.544, CI 1.599–19.228, P = 0.007) was a significant risk factor. Two patients (2.1%) showed bradycardia (asymptomatic, intervention not indicated) on Day 3 and recovery on Day 5. Limitations for this study included the difficulty distinguishing AE from worsening of COVID-19, thus we documented as clinical events. Conclusions For 24 h after infusion of casirivimab-imdevimab, COVID-19 patients with increased white blood cell counts may be predisposed to temperature elevation more than 1.0 degrees centigrade, as may bilateral ground glass opacity to lowered oximetry. Thus, patients with leukocytosis and bilateral ground glass attenuation may need precaution for transient fever and hypoxia, respectively.