BACKGROUND Many studies have assessed the risk factors for adverse drug reactions (ADRs) in elderly patients. However, most of these studies have focused on risk factors for ADRs, not serious ADRs (s-ADRs). s-ADRs are commonly found in hospitalized patients. s-ADRs warrant imminent but thorough investigations, given their critical impact on patient health. Therefore, this retrospective study aimed to assess the associated risk factors for s-ADRs in elderly hospitalized patients. METHODS In-patients aged >65 years having ADRs during hospitalization at a university hospital in Korea between 2010 and 2012 were included. Medical professionals spontaneously reported ADRs using an electronic submission system at the study hospital. Further, all descriptions of ADRs were characterized and categorized through the screening of electronic medical records. We compared the characteristics of patients having s-ADRs with those of patients not having s-ADRs. RESULTS There were 353 cases of ADRs, 67 of which were s-ADRs. Patients taking more than eight concomitant drugs showed the highest odds ratio (OR, 11.99; 95% confidence interval [CI], 3.42-42.03). The ratio of aspartate aminotransferase (AST)/alanine aminotransferase (ALT) was also significantly related to s-ADRs (OR, 2.78; 95% CI, 1.33-5.81). The use of antibiotics (OR, 2.39; 95% CI, 1.13-5.02) and antineoplastics (OR, 4.17; 95% CI, 1.09-15.94) were significant risk factors. CONCLUSIONS Our findings highlight the importance of polypharmacy. Liver function tests (AST/ALT ratio) must be monitored carefully within high-risk groups for ADRs.