From November 1990 to February 1991 was had, in the Emergency Unit of Clinical University Hospital San Carlos, Madrid, 13 documented cases of intoxication due to Carbon Monoxide (CO), in patients with unspecific cephalalgia and asthenia and a possible CO source (total 19 affected people living together, because some of them did not came to hospital to consult). We consider that CO intoxication shall be always be beared in mind when doing the diagnosis at the Emergency Unit, especially during winter months, and that appropriate resources should be available to perform promptyl the analytical confirmation.