Adequate categorization of adverse drug reaction (ADR) causality may contribute to its early recognition in the assessment of ADR. The method proposed by World Health Organization and Uppsala Monitoring Center (WHO-UMC system) is universally used for causality assessment. We sought to evaluate ADR causality in hospitalized patients using WHO-UMC system. ADR preventability was also evaluated in this study. We reviewed cases of ADR identified in a Brazilian university hospital. Three healthcare professionals adjudicated, independently, each ADR reported using WHO-UMC method. Overall, 90 ADR involving 64 patients were evaluated and adjudicated as possible in 47 (52.2%) cases. Slight concordance was found in the agreement analyses, with kappa range of 0.10-1.18, indicating poor reproducibility of WHO-UMC system. Our results suggest the involvement of interdisciplinary teams could improve the quality of the monitoring process of ADR. A great number of preventable ADR reinforce the need for improvements in pharmacovigilance strategies in hospital settings.