The effect of computerized cranial tomography (CCT) on physicians' diagnostic and therapeutic plans in 194 patients scanned consecutively during a 17-day period at the Massachusetts General Hospital was studied. Use of CCT substantially decreased the perceived need for other neurodiagnostic tests in this patient population. There was a reduction of a least 41% in radionuclide scans, 52% angiograms, and 73% in pneumoencephalograms compared to the needs projected had CCT not been available. Therapy was altered after 37 scans in 36 patients (19% of those examined): new treatment was begun after 15 scans, previously planned therapy became more precise after 9 scans, treatment was found to be unnecessary after 7 scans, and treatment was abandoned as worthless after 6 scans.