In 1983 the North American Hyperbaric Center (affiliated with Bronx Municipal Hospital Center) was designated to provide Hyperbaric oxygen (HBO) for carbon monoxide (CO) patients meeting Emergency Medical System (EMS) criteria: 1. Unconscious or CNS derangement, any carboxyhemoglobin level [( COHb]); 2. [COHb] 25% or more; 3. Pregnant, any [COHb]. Through 1984, 39 CO patients received HBO; in 1985 81 were treated including 8 pregnant and 16 pediatric. CO sources were: fire, 43; heater, 21; engine, 17. 42 of 59 acute patients were initially in coma; 16 required CPR. Time to HBO averaged 4.5 hrs. Mean hospital [COHb] was 31% for acute, 28% for subacute/chronic, 15% for pregnant; mean maximum [COHb] was estimated: 51% for acute, 40% for subacute/chronic patients. Mean half life for [COHb] was 2.1 hrs. HBO typically was 46 min at 3ATA (2 [COHb] half lives), presented few problems, and gave rapid clinical improvement. 13 of 19 patients comatose before HBO were responsive after HBO (mean [COHb] was 1.8%). Four pediatric deaths occurred; brain damage was noted in two other patients. EMS efforts to make HBO available for CO is a success.