IN RECENT YEARS, numerous reports have appeared in the literature on the use of diagnostic paracentesis, particularly in cases of abdominal trauma and acute disease requiring surgery. In these reports, complication rates have been low and the use of this technique has been strongly advocated. Recently, we have encountered a patient in whom an erroneous diagnosis was made due to an abdominal paracentesis. It is our purpose to alert physicians to an error in technique that may lead to a false diagnosis and subsequent unnecessary laparotomy. Report of a Case A 20-year-old white man (BMHC 365880) was admitted four hours after being involved in an automobile accident during which he was thrown against the steering wheel of his car. He had lost consciousness for approximately two minutes. Following the accident, the patient developed pain in the right upper abdominal quadrant and right anterior parts of the chest. He vomited once