An earlier study showed that the prothrombin time determination is overutilized as a routine admission laboratory test. In the present study the investigator evaluated the effectiveness of an educational program in decreasing utilization of the prothrombin time determination by house staff physicians at a teaching hospital. Six months after the educational program, use of the prothrombin time determination as a routine admission test had decreased significantly from 87 percent to 55 percent. Eighteen months after the educational program, the use of this test by the study group had returned to initial levels. These results emphasize the need to measure the long-term outcome of educational programs aimed at modifying the clinical behavior of physicians and to institute follow-up programs to ensure that an educational effort will have a lasting effect.