Corticosteroids are often administered prophylactically to attenuate the inflammatory response associated with cardiac surgery using cardiopulmonary bypass (CPB). However, the efficacy and safety profile of corticosteroids remain uncertain. The primary aim of this systematic review and meta-analysis was to investigate the effect of corticosteroids on mortality in adult cardiac surgery using CPB. Secondary aims were to examine the effect of corticosteroids on myocardial adverse events, pulmonary adverse events, atrial fibrillation, surgical site infection, gastrointestinal bleeding and duration of stay in the intensive care unit and hospital. Randomized controlled trials (RCTs) were systematically searched in electronic databases (MEDLINE, EMBASE, CINAHL, CENTRAL and Web of Science) from their inception until March 2019. Observational studies, case reports, case series and literature reviews were excluded. Sixty-two studies (n = 16 457 patients) were included in this meta-analysis. There was no significant difference in mortality between the corticosteroid and placebo groups [odds ratio (OR) 0.96, 95% confidence interval (CI) 0.81-1.14; P = 0.65, participants = 14 693, studies = 24, evidence of certainty: moderate]. Compared to those receiving a placebo, patients who were given corticosteroids had a significantly higher incidence of myocardial adverse events (OR 1.17, 95% CI 1.03-1.33; P = 0.01, participants = 14 512, studies = 23) and a lower incidence of pulmonary adverse events (OR 0.86, 95% CI 0.75-0.98; P = 0.02, participants = 13 426, studies = 17). The incidences of atrial fibrillation (OR 0.87, 95% CI 0.81-0.94; P < 0.001, participants = 14 148, studies = 24) and surgical site infection (OR 0.81, 95% CI 0.73-0.90; P < 0.001, participants = 13 946; studies = 22) were all lower in patients who were given corticosteroids. In the present meta-analysis of 62 RCTs (16 457 patients), including the 2 major RCTs (SIRS and DECS trials: 12 001 patients), we found that prophylactic corticosteroids in cardiac surgery did not reduce mortality. The clinical significance of an increase in myocardial adverse events remains unclear as the definition of a relevant myocardial end point following cardiac surgery varied greatly between RCTs.