To investigate occult carbon monoxide poisoning in patients with neurologic illness, we prospectively studied 168 patients who presented to the emergency department between December 1987 and February 1988 with neurologic symptoms for evidence of carbon monoxide exposure. Patients with known carbon monoxide poisoning were excluded. The mean carboxyhemoglobin level was 3.1 percent; there were no significant differences in carboxyhemoglobin between categories of neurologic illness (F(5,162) = 1.35; p less than 0.25). Five patients (3 percent) had a carboxyhemoglobin greater than 10 percent, with levels ranging from 11.7 percent to 29.5 percent. After controlling for the effects of active and passive exposure to cigarette smoke, problems with the home heating system (odds ratio 9.6; p less than 0.03) and the presence of cohabitants with concurrent headache or dizziness (odds ratio 21.6; p less than 0.0001) were associated with an increased risk of a carboxyhemoglobin greater than 10 percent. A rule for obtaining carboxyhemoglobin tests only on patients who used gas stoves for heat or who had symptomatic cohabitants would have correctly identified all patients with carboxyhemoglobins greater than 10 percent, correctly excluded 77 percent of patients with lower levels, and eliminated the need for testing in 75 percent of cases. We conclude that unrecognized carbon monoxide poisoning occurs in a small but important fraction of patients with wintertime neurologic illness and can be identified by a characteristic risk factor profile.