The determinants of the frequency of Canadian malpractice claims, the proportion of claims that result in payment, and the severity of these claims are examined. Inter-specialty variation in the frequency of malpractice claims is almost entirely related to the differential performance of major surgery. Various legal doctrines concerning both compensation and liability appear responsible for approximately half of the upward trend in the propensity to initiate malpractice litigation. We believe that the remaining explanations for growth in claims frequency are changes in social attitudes toward risk-bearing, increasing social distance between patients and physicians, and innovations in medical technology.