Adverse drug reactions (ADRs) are an important health issue. While prevalence and risk factors associated with ADRs in the general adult population have been well documented, much less is known about ADRs in the elderly population. The aim of this study was to review the published literature to estimate the prevalence of ADRs in the elderly in the acute care setting and identify factors associated with an increased risk of an ADR in the elderly. A systematic review of studies published between 2003 and 2013 was conducted in the Cochrane Database of Systematic Reviews, EMBASE, Google Scholar and MEDLINE. Key search terms included: “adverse drug reactions”, “adverse effects”, “elderly patients and hospital admission”, “drug therapy”, “drug adverse effects”, “drug related”, “aged”, “older patients”, “geriatric”, “hospitalization”, and “emergency admissions”. For inclusion in the review, studies had to focus on ADRs in the elderly and had to include an explicit definition of what was considered an ADR and/or an explicit assessment of causality, and a clear description of the method used for ADR identification, and had to describe factors associated with an increased risk of an ADR. Fourteen hospital-based observational studies exploring ADRs in the elderly in the acute care setting were eligible for inclusion in this review. The mean prevalence of ADRs in the elderly in the studies included in this review was 11.0% (95% confidence interval [CI]: 5.1%–16.8%). The median prevalence of ADRs leading to hospitalization was 10.0% (95% CI: 7.2%–12.8%), while the prevalence of ADRs occurring during hospitalization was 11.5% (95% CI: 0%–27.7%). There was wide variation in the overall ADR prevalence, from 5.8% to 46.3%. Female sex, increased comorbid complexity, and increased number of medications were all significantly associated with an increased risk of an ADR. Retrospective studies and those relying on identification by the usual treating team reported lower prevalence rates. From this review, we can conclude that ADRs constitute a significant health issue for the elderly in the acute care setting. While there was wide variation in the prevalence of ADRs in the elderly, based on the findings of this study, at least one in ten elderly patients will experience an ADR leading to or during their hospital stay. Older female patients and those with multiple comorbidities and medications appear to be at the highest risk of an ADR in the acute care setting.