BACKGROUND: Worldwide, the emergence of super-ageing societies has increased the number of older people requiring support for daily activities. Many elderly residents of nursing homes (NHs) take drugs to treat chronic conditions; however, there are few reports of medication safety in NHs, especially from non-western countries. OBJECTIVE: We examined the incidence and nature of adverse drug events (ADEs) and medication errors (MEs) in NHs for the elderly in Japan. DESIGN, SETTING, AND PARTICIPANTS: The Japan Adverse Drug Events Study for NHs is a prospective cohort study that was conducted among all residents, except for short-term admissions, at four NHs for older people in Japan for 1 year. MEASUREMENTS: Trained physicians and psychologists, five and six in number, respectively, reviewed all charts of the residents to identify suspected ADEs and MEs, which were then classified by the physicians into ADEs, potential ADEs and other MEs after the exclusion of ineligible events, for the assessment of their severity and preventability. The kappa score for presence of an ADE and preventability were 0.89 and 0.79, respectively. RESULTS: We enrolled 459 residents, and this yielded 3315 resident-months of observation time. We identified 1207 ADEs and 600 MEs (incidence: 36.4 and 18.1 per 100 resident-months, respectively) during the study period. About one-third of ADEs were preventable, and MEs were most frequently observed in the monitoring stage (72%, 433/600), with 71% of the MEs occurring due to inadequate observation following the physician's prescription. CONCLUSION: In Japan, ADEs and MEs are common among elderly residents of NHs. The assessment and appropriate adjustment of medication preadmission and postadmission to NHs are needed to improve medication safety, especially when a single physician is responsible for prescribing most medications for the residents, as is usually the case in Japan.