The authors report an observational study of adverse event incident reports collected from 1993 through 1995 from a nonrandom sample of general practitioners in Australia. Of the 805 incidents reported, 76% were qualitatively judged to be preventable and 27% to have the potential for severe harm. The authors provide a detailed breakdown of the types of incidents, factors that mitigated the harm caused by the incidents, and demographics of the affected patients. The authors classify the underlying causes of the events in four categories—communications problems, procedural problems (which include cognitive errors), clinical problems, and external problems—based on qualitative assessment of the reports.