The authors describe the cases of two mechanically ventilated septic patients, in whom developed during their illness with the signs of severe tetraparesis critical illness neuropathy. In both cases the first sign of the neuropathy was the respirator dependency despite the improving neurological status. The severe general condition of the patients and the administered sedatives obscured the neurological signs for a long time, and only the development of a severe tetraparesis raised the suspicion of the critical illness neuropathy. In the first case the diagnosis was made by exclusion, but in the second case it was proved with the help of EMG and the histological examination of the m. biceps brachii. The authors discuss the clinical relevance, neurological and electrophysiological signs and the problems of the differential diagnosis of this disease, which is quite common among the critically ill, septic patients.