Headache and dizziness are early symptoms of carbon monoxide poisoning, occurring at carboxyhemoglobin levels of greater than 10 percent. Previously, it was shown that among patients presenting to an emergency department during the winter with headache or dizziness, an algorithm for obtaining carboxyhemoglobin levels on patients who used gas stoves for heating purposes or who had similarly affected cohabitants correctly identified all patients with carboxyhemoglobin levels greater than 10 percent. To test the validity of this retrospectively derived rule, 65 patients were studied who were unaware of any carbon monoxide exposure and who presented during the winter of 1986-1987 with headache or dizziness. The algorithm correctly identified three of four patients with carboxyhemoglobin levels greater than 10 percent (sensitivity = 75 percent) and correctly excluded 45 of 61 patients with lower levels (specificity = 74 percent). The presence of symptomatic cohabitants alone was an equally sensitive (75 percent) but more specific (90 percent) marker for elevated carboxyhemoglobin levels. When data from the two cohorts were combined, stepwise multiple regression identified number of cigarettes smoked daily (F = 8.66) and concurrently symptomatic cohabitants (F = 34.71) as significant predictors of the carboxyhemoglobin level. It is concluded that a retrospectively derived rule correctly identified most cases of occult carbon monoxide poisoning when applied prospectively, and that the presence of similarly affected cohabitants was the most reliable marker for a carbon monoxide-mediated illness.