Among 1,001,000 ASA Physical Status I and II patients (a subset of the 1,329,000 anesthetics administered from 1976 through mid-1988 in the nine component hospitals of the Harvard Department of Anaesthesia), there were 11 major intraoperative accidents solely attributable to anesthesia (five deaths, four cases of permanent CNS damage, and two cardiac arrests with eventual recovery) among the 70 cases reported to the insurance carrier. Review of these accidents revealed that unrecognized hypoventilation was the most common cause (seven cases). These seven accidents and one other due to discontinuation of inspired oxygen in all likelihood would have been prevented by appropriate response to earlier warnings generated by the “safety monitoring” principles mandated by the Harvard minimal monitoring standards. Analysis suggests capnography (although not mandated) would be the best monitor of ventilation. An important associated issue was the apparent inadequacy of supervision of residents and C.R.N.A.s. The eight preventable accidents represent 88% of the projected insurance payout. Only one accident occurred after the 1985 adoption of the standards (in the month following their implementation). From that time through mid-1988, there have been 319,000 anesthetics without a major preventable intraoperative injury. Although not statistically significant, the accident rate in the target population of healthy people is reduced more than threefold. This and the case analyses support the contention that nearly all the inevitable mishaps (technical or from errors in judgement) that occur during anesthesia can be identified through safety monitoring early enough to prevent most major patient injuries. This improved clinical outcome should lessen the medical-legal and malpractice insurance burdens of anesthesiologists.