Cranial surgery as practised by the prehistoric and present-day traditional medicine man is the oldest known and one of the highest surgical achievements in the history of medicine. This study examines morbidity and mortality of this ancient surgical procedure by re-examining and reviewing reports of 71 completely trepanned skulls from pre-Columbian Peru by three techniques: gross visual observation and palpation; radiography; and CT. The specimens are located in the San Diego Museum of Man and are part of the Hrdlicka collection. Clear distinction can be made by visual observation between skulls with bone regeneration and those without; that is, between patients who survived long enough to allow bone healing and those who died immediately or within a few days of the operation. 64.8 per cent of skulls had complete healing, 12.7 per cent had partial healing and 22.5 per cent had no evidence of healing. Thus, a post-operative survival rate of 77.5 per cent is noted by gross examination. The meaning of completely trepanned skulls with no evidence of healing is questionable, however, some unhealed skulls may represent post-mortem trepanation, suggesting a lower surgical mortality. Both radiography and CT scan demonstrate with considerable accuracy the presence or absence of new bone formation on trepanned skulls. One skull demonstrated evidence of partial bone healing by CT and radiography but not by our gross examination or by that of some anthropologists. Evidence of osteomyelitis was illustrated by CT scan but not by radiography.