Objective: Organizational factors affecting working conditions for health care workers have received significant attention as latent causes of medical errors. Little is known, however, about the risks associated with transient, or changing working conditions. The purpose of this study was to identify specific transient modifiable working conditions in the hospital environment that serve as triggers for medical errors. Methods: a case-crossover design was used to study proximate causes of medical errors. Nursing personnel directly involved in a medical error were interviewed within two weeks of the error. Specific attributes of working conditions were assessed at the time immediately preceding the error and at times when no error occurred. Variables examined include subjects' perceptions of work pace, patient census, patient acuity, teamwork, and distractions. Preliminary results are based on 112 interviews completed to date. Results: Subjects were more likely to describe their work environment as more hectic and reported increased distractions and feelings of fatigue during the 30 minutes prior to the error occurring as compared to the entire error shift. Subjects were more likely to report missing important patient information, having higher acuity patients, and experiencing significant events on shifts when errors occurred, as compared to shifts when no errors occurred. Conclusions: These preliminary results suggest that working conditions immediately preceding the medical error and on the error shift differed from times when no error occurred, suggesting that transient working conditions may contribute to medical errors. Changes to the work environment such as improving the transmission of important patient information may help reduce the occurrence of medical errors.