BACKGROUND: Communication failure is a common root cause of preventable medical errors affecting hospitalized patients. A study was conducted to determine the reasons for calls made by nurses working on the general medical wards to on-call physicians from 6:00 P.M. to 7:00 A.M. METHODS: A retrospective review was performed of a random sample of 500 inpatients admitted to general medical wards at an urban public teaching hospital in Houston between January 1, 2000, and February 28, 2003. RESULTS: In 139 (47%) of 293 medical records there were 304 documented calls. The majority of calls (81%) took place between 6:00 P.M. and 2:00 A.M., with peak call volume between midnight and 2:00 P.M. Patients with one or more calls had an average of 2.2 calls during their stay. Ten categories accounted for 65% of all the nurse calls. In 44% of calls, physicians responded by ordering a medication. DISCUSSION: Communication between floor nurses and on-call physicians might be improved by several interventions. Because 10 reasons accounted for 65% of after-hours calls, protocols could allow nurses to resolve some acute problems without physician involvement. For example, appropriate standing orders (e.g., PR.N. medications) may prevent some calls. In addition, sign-out procedures can be tailored to address common problems that are likely to require future telephone communication. With efforts to change error-prone systems, it seems prudent to focus on after-hours coverage.