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INTRODUCTION: Chronic intoxication due to carbon monoxide (CO) is a condition in which the frequency is underestimated since the clinical features are nonspecific. It is important since serious neurological sequelae may result: an extrapyramidal syndrome, dementia or a vegetative state. The diagnosis is established when there are venous blood levels of carboxyhemoglobin greater than 10%. Early treatment with hyperbaric O2 rapidly improves the symptoms and avoids the development of neurological sequelae. CLINICAL CASE: We report the case of a 30 year old woman who was admitted to hospital for investigation into two episodes of loss of consciousness with vomiting and subsequent somnolence not accompanied by other neurological sequelae. For the previous eight months she had had daily pulsatile biparietal headaches, sometimes accompanied by a fainting feeling, nausea and vomiting. During her admission she remained asymptomatic and all neurophysiological, cardiological and neuroimaging studies were normal. The day after medical discharge she returned with the same clinical condition. A woman who lived with her and accompanied her also complained of similar symptoms. Venous gasometry showed raised levels of carboxyhemoglobin in both the patient (30.4%) and her companion (31.2%). Treatment with hyperbaric 100% O2 reverted the symptoms within a few hours. Later studies showed that the domestic gas burner was faulty. CONCLUSIONS: Occult CO intoxication causes headache which often requires differential diagnosis from psychiatric disorders and episodes of migraine. In patients with refractory nonspecific headache, irregular course and systemic symptoms, usually in winter, CO intoxication should be considered to be a possible cause. Diagnosis is based on finding venous blood levels of carboxyhemoglobin of over 10%. Early treatment avoids lesion of the globus pallidus and irreversible extrapyramidal sequelae.