Adverse drug reactions (ADRs) are cause of significant morbi-mortality They are between the fourth and sixth cause of mortality in developed countries and cause nearly 12% of hospitalizations. The objective of this publication was to analyze the incidence of ADRs in a tertiary care hospital in Buenos Aires City. The hospital phamacovigilance database for the period June 2008- February 2012, was analyzed. The Naranjo score was applied to assess drug causality. We consider serious an ADRs when it potentially compromised life, induced hospitalization or prolonged it, caused discapacity, teratogenesis or death. In this period, a total of 2420 ADRs were detected: 469 (19.38%; CI 95%: 17.80 - 20.95) were serious, mainly because they induced hospitalization (n = 287). There were 14 ADRs- related deaths. Cardiovascular and neuropsychiatric drugs, antibiotics and corticoids were those most frequently related to toxicity. Endocrine-metabolic disorders, hepatotoxicity, nephrotoxicity and pharmacodermy were the most frequently involved. Among the ADR most frequently associated to hospitalization were Immunosuppressant-associated severe infections and upper gastrointestinal bleeding related to oral anticoagulants and non steroids anti-inflammatory drugs. The ADRs incidence in hospitalized patients and ADRs related hospital admissions were considered relatively high. Drugs involved were similar to those reported in the international bibliography except for the higher incidence of immunosuppressants related admissions here observed.