The quality of primary medical care was assessed by studying the events leading to 686 emergency admissions of patients from our hospital-based primary care practice. Independent physician reviewers determined that 59 (9 percent) of the admissions were potentially preventable; 40 were due to iatrogenic factors including inadequate follow-up and adverse drug reactions, 12 were due to lack of patient compliance, and seven were due to both iatrogenesis and noncompliance. Adverse drug reactions were the most common cause of iatrogenesis, and warfarin was the drug that most commonly caused an adverse reaction. Inadequate follow-up of abnormal physical findings, symptoms, and laboratory test results was also important. Patients with preventable admissions had more medical diagnoses (4.9 versus 4.1, p less than 0.01), were prescribed more medications (4.5 versus 3.7, p less than 0.01), and were older (66.5 years versus 60.2 years, p less than 0.01) than patients whose admissions were not preventable. It is concluded that a small percentage of emergency hospitalizations may be preventable and that systematic review of emergency hospitalizations may provide a means of measuring the quality of primary medical care.