The costs and performance of the Cornwall and Isles of Scilly Ambulance Service Helicopter Unit (CASHU) air ambulance (A/A) was assessed during 1 year (1 December 1990-30 November 1991). The helicopter, with a crew of one pilot and two paramedics, commenced 797 patient-related missions during the year, 554 (69.5 per cent) as an emergency response ambulance. Eighty-one (15 per cent) of the emergency missions were aborted with no patient attended. Compared with the activity of ground ambulances, the A/A attended more accidents (66.5 per cent vs 40.8 per cent), and more incidents occurring outside in fields, or on cliffs or beaches (47.1 per cent vs 9.5 per cent). In its role as an emergency response vehicle the helicopter performance was not better than that of ground ambulances. Activation times, response times and on-scene times were all longer on average than for ground ambulances. However, transport times to hospital were sometimes much faster by helicopter. Although total times from 999 call to arrival in hospital were similar in the two cohorts, this was principally because longer distances were involved in the helicopter missions. Standardized for distance the A/A was, on average, 10 min faster than predicted for a land ambulance. The estimated direct costs of CASHU were 595,000 pounds for the year, and the costs of carrying out the helicopter missions by road have been estimated to be 97,805 pounds. The consequent marginal cost of around 500,000 pounds must be set against any health benefits accruing from the performance improvements for patients.