Patients unidentified at the time of admission to urban emergency departments are a group about whom little is known. To determine the medical diagnoses and outcomes of these "John" and "Mary Does", we reviewed emergency department charts for these patients admitted from January 1 to December 31, 1988. During this period there were 344 initially unidentifiable patients, for 0.44% of all visits. Age was 36.9 ± 15.6 years (mean ± SD); 71% were male. All patients had one or more of the following diagnoses, with mortality highest for cardiopulmonary arrest (n = 42, mortality = 100%), followed by major trauma (163, 68%), drug overdose (27, 41%), miscellaneous medical conditions (11, 18%), neuropsychiatric disorders (59, 12%), acute alcohol intoxication (62, 0%), and seizures (13, 0%). Overall mortality was 47%. Identification was made prior to hospital discharge in 92% of cases. In this group, the most cummon sources of information were the patient (38%), family (19%), or documents eventually found on the person or in belongings (4%). Survivors were much more likely to be identified than those who died (99% versus 84%, P < .0001). These observations describe a John Doe syndrome in patients whose identity is obscured by critical illness, the effects of drugs or alcohol, or neuropsychiatric disease. Economic privation is a major underlying risk factor. © 1992.