Hospitals' patterns of ancillary service use were examined to determine whether new technologies replace older, more outmoded technologies, and to explore the factors associated with adoption of newer services and abandonment of older services. Annual inpatient use of five pairs of ancillary services was measured for 1978 through 1980 at 63 hospitals in five regions. The diagnostic test pairs consisted of one well-established diagnostic test and one newer service that could largely substitute for the older one and included (1) oral cholecystogram and gallbladder ultrasound; (2) brain scan and computed tomographic head scan; (3) skull roentgenogram and brain scan; (4) bone survey and bone scan; and (5) blood type/cross and type/screen. Use of gallbladder ultrasound increased significantly after its adoption, with small decreases in the use of oral cholecystogram, its paired test. For the other newer tests examined, increased use was not accompanied by significantly decreased use of the paired older service. The strongest predictors of change in patterns of test use were hospital size, number of residencies, occupancy, urban location, and the proportion of specialists on staff. We conclude that diffusion of new diagnostic services occurs gradually and often without concomitant decrease in older, outmoded services; new services generally seem to complement rather than substitute for older ones. Larger hospitals with a greater teaching commitment make a faster transition to the use of new technologies and the abandonment of older ones.