Colleague and PaperChase are the two most widely used computer systems designed specifically for clinicians and scientists who wish to search the National Library of Medicine's MEDLINE data base of references to the biomedical literature. The present study compares the performance of these two systems. Two matched groups of second-year medical students each received 3 hr of instruction, one group in Colleague, the other in PaperChase. Each student then attempted 10 test searches. The next day the groups were reversed, and each student attempted 5 additional searches. During 3 1/2 hr allocated for searching, users of Colleague attempted 64 test searches and retrieved 326 target references; users of PaperChase attempted 78 searches and retrieved 496. Users of Colleague took a mean of 2.2 min and spent a mean of $1.20 to find each target reference; users of PaperChase took 1.6 min and spent $0.92. We conclude that after limited training, medical students find more references faster and at lower cost with PaperChase than with Colleague.