A questionnaire survey was conducted with questions from the 12-item General Health Questionnaire, among others, targeting 4,407 nurses in 8 general hospitals in Japan, in the hope of improving the work environment of nurses and to provide data that will allow a discussion of the measures necessary for preventing medical errors, thus improving occupational health. For each type of accident, the percentage of those who had made medical errors was significantly higher for the "mentally in poor health" group than for the "mentally in good health" group (p<0.0001). The percentage of nurses in the "mentally in good health" and "mentally in poor health" groups who had experienced occupational accidents over the past 12 months (i.e., whether they were "with errors" or "without errors") was calculated for each of the following four types of medical accident: (1) drug-administration errors, (2) incorrect operation of medical equipment, (3) errors in patient identification, and (4) needlestick injuries. For each type of accident, the percentage of those who had made medical errors was significantly higher for the "mentally in poor health" group than for the "mentally in good health" group (p<0.0001). Multiple logistic regression analyses revealed significant associations between experience of medical errors in the past 12 months and being mentally in poor health, with night or irregular shift work, and age.