Five hundred seventy-seven institutions examined how accurately physicians' test orders on inpatients were transmitted to the laboratory. Written orders could be found on laboratory requisitions or the medical record for 97.5% of 224,431 completed tests (median institution = 99.3%). Participants indicated that entry of extra tests into a hospital computer was the most common reason for completing unordered tests. In a multivariate analysis, factors associated with completing unordered tests were the lack of a policy requiring nursing staff to recheck computer orders against the medical record, average census of 301-450 patients, College of American Pathologists accreditation, and the use of preprinted "checkoff" order forms. Overall, 97.1% of 225,457 test orders were completed by the laboratories (median institution = 98.1%). Factors associated with not completing ordered tests were the lack of a policy requiring staff to check computer orders, teaching hospital status, and urban hospital location. Several interventions commonly thought to improve communication of orders were not found to affect performance. These results indicate that many institutions have a problem accurately transmitting test orders to their clinical laboratories.