To assess the degree to which use of hospital tests and procedures changed over a five-year period, we studied 1203 patients who were hospitalized at the University of California. San Francisco, in either 1972 or 1977 with one of 10 diagnoses: acute asthma, acute myocardial infarction, lung cancer, respiratory-distress syndrome of the newborn, cataract excision, cesarean section or vaginal delivery, kidney transplantation, stapedectomy, or total hip replacement. After careful adjustment for case severity, the total number of tests and procedures per hospital stay was found to be relatively unchanged over the five-year period for most but not all the diagnoses. However, the use of certain new diagnostic procedures (such as determination of arterial blood gases, ultrasonography, fetal monitoring, and radioisotope scanning) did increase significantly. Although generalization from these limited observations must be cautious, the data suggest that a "technology imperative" may apply more to the introduction of new technologies than to the expanding use of older, established tests and procedures. Effective cost-containment strategies must recognize the complexities of technology use among different diagnoses.