Progressive mental deterioration in chronic pulmonary patients during periods of acute respiratory decompensation is usually suspected to be due to CO2 intoxication. A study of 50 consecutive patients clinically labelled as "CO2 intoxication or narcosis syndrome" revealed that 82% were hypercapneic while 18% clinically mimicked the syndrome but had in fact normal or low PCO2 values. The degree of mental impairment did not correlate well with the degree of any isolated laboratory abnormality. The evolution and prognosis were mostly related to the very frequent extrapulmonary complications, particularly heart failure (82%), shock (55%) and gastrointestinal bleeding (33%). The overall death rate was 36% and in only 3 patients was the death attributed solely to progression of hypoventilation. Severe hypoxemia of less than 80% carried a high mortality rate. It is concluded that the clinical syndrome of "CO2 intoxication" includes a wide range of synptoms, degree of hypercapnia and complications. In the context of the acute syndrome, one cannot easily separate the relative deleterious effects of hypercapnea, hypoxemia and extrapulmonary factors.