Chronic exposure to low levels of carbon monoxide can cause vague symptoms that are easily mistaken for other common illnesses. During the past 5 years, three families have contacted the Wisconsin Division of Public Health to report illnesses that may have been caused by chronic exposure to carbon monoxide. Members of these families were diagnosed with a variety of conditions including chronic fatigue syndrome, depression and influenza. Carbon monoxide exposure was not suspected as a cause of these illnesses until heating contractors discovered that gas appliances in these families' homes were not properly vented. These cases serve as reminders that carbon monoxide exposure should be considered in the differential diagnosis of patients who present with chronic symptoms of headache, fatigue, dizziness, nausea and mental confusion--especially when these symptoms onset during the winter heating season.