Two cases of severe chronic respiratory insufficiency in severely acute phase with disturbances in acid base balance characterized by serious gaseous acidosis are reported. Therapy was based essentially on controlled ventilotherapy using an iron lung and considerable improvements were achieved clinically with practically total normalization of the acid base imbalance after only a few hours of treatment. In the following days, however, a picture of metabolic alkalosis established itself and this is discussed and interpreted as an expression of post-hypercapnic hypochloraemia.