Clinical librarian services provide highly specific, case-related medical literature searches in a manner that permits the information obtained to influence ongoing case management. We have measured the impact of such services on actual case management and on diagnostic thinking in a random sample of 50 searches over five months provided to house officers on medical and pediatric services of a university hospital. Patient management was affected in 20% of cases, and diagnostic thinking was influenced in an even higher percentage of instances. In comparison with information generated by other ancillary services such as clinical laboratories, clinical librarian services appear to be highly effective at an acceptable cost. Since the information generated by such searches may often result in further cost savings to patients, the potential overall efficiency of such services may be great.