As Dr Blumenthal indicates (June 2001 JRSM, pp. 270-272), much is known about the acute effects of carbon monoxide poisoning and quite a lot about the long-term sequels of such poisoning. However, much less has been published about the effects of chronic low-grade exposure to carbon monoxide poisoning. This is often extremely insidious and is usually not detected during the exposure but only later after realization that a gas-fired appliance is defective. During the exposure the common symptoms are headache, nausea and weakness; vomiting and dizziness may also be complained of. Lethargy can be a particularly severe symptom and may result in a misdiagnosis of chronic fatigue syndrome. Our experience encompasses over 30 such cases. The psychiatric effects of chronic exposure include irritability, depression and lability of mood. Neurological signs are usually absent. The neuropsychological pattern on detailed testing is impairment of attention, short-term memory and executive functioning. We wish to draw doctors' attention to this syndrome as it is easily overlooked.