We performed a prospective study to determine the incidence of missed, potentially toxic acetaminophen poisoning in all patients with any type of intentional drug ingestion presenting to two large, county hospital emergency departments. Of 486 patients with drug ingestion seen during a five-month period, 114 (23.5%) were suspected of having ingested acetaminophen. Of these, 71 (62.3%) had insignificant acetaminophen levels (false-positive history). Of the 365 patients who were not suspected of having acetaminophen ingestion, seven patients (1.9%) were found to have elevated levels on rapid screening (false-negative history). Of these, only one patient had a potentially hepatotoxic level. We conclude that the incidence of missed, potentially serious acetaminophen overdose is very low in our study population.