Background: Limited data regarding knowledge and factors related to understanding the adverse drug reaction (ADR) reporting system of health care professionals are available in Japan. Objective: The objective of this study was to identify factors related to understanding the ADR reporting system in Miyagi Cancer Center and to find ways to increase the number and quality of ADR reports. Methods: Self-administered questionnaire surveys were administered before and after the educational meeting among health care professionals who were working in our hospital during the study period. Subanalyses restricted to nurses were also performed. Main Outcome Measure: Understanding ADR reporting system among healthcare professionals. Results: The percentage of respondents who understood the ADR reporting system in the questionnaire after the educational meeting was significantly higher than that in the questionnaire before the educational meeting. In the questionnaire after the educational meeting, multivariate logistic regression analysis found that having over 30 years of practical experience (odds ratio [OR], 3.852; 95% confidence interval [CI], 1.228-12.081 for 20-29 years, 7.695; 1.650-35.881 for over 30 years), being a physician (8.071; 1.923-33.878), being a pharmacist (18.357; 3.847-87.585), and participating in the educational meeting (5.111; 1.700-15.365) were factors associated with understanding the ADR reporting system. Multivariate logistic regression analysis of the questionnaire results before the educational meeting among nurses showed that working at outpatient departments (8.330; 3.008-23.069) was significantly and independently associated with understanding the ADR reporting system. Conclusions: The present study demonstrated that many years of practical experience, profession (physicians, pharmacists), and educational interventions were associated with good understanding of the ADR reporting system among health care professionals.