To evaluate the importance of diagnoses undetected before cardiac arrest in the hospital, we studied autopsy findings on 130 patients who died after an attempt at cardiopulmonary resuscitation (CPR). We also studied the complications that occurred in these patients as a result of CPR. Twenty-one percent of the patients had at least one complication as a result of CPR. Patients resuscitated on the wards were more likely to have a complication than those treated in the intensive care unit. This suggests that more proficient technique in CPR may reduce morbidity from this procedure. In 14% of the cases, there was a major missed diagnosis. The two diseases most frequently undetected clinically were ischemic bowel and pulmonary embolus, which together accounted for 89% of all major missed diagnoses discovered at autopsy. We conclude that diseases that require a high prior clinical suspicion (bowel infarction and pulmonary embolus) are common accompaniments of cardiac arrest in the hospital. Consideration of these diagnoses in critically ill patients may prevent future cardiac arrest and death from pulmonary embolus and ischemic bowel.