The authors report three cases of neuromuscular deficits occurring after a variable time of mechanical ventilation in ICU patients suffering from acute respiratory failure requiring sedation and neuromuscular blockade by pancuronium bromide. The clinical features of these deficits were similar : flaccid quadriparesia slowly reversible without sensory loss and difficult weaning from ventilation for two patients. The paraclinical investigations (measurements of creatine kinase, electrophysiological examination) permitted to distinguish two diseases: critical illness polyneuropathy (in patient with ARDS) and acute myopathy (in other patients with severe asthma) caused by corticosteroids and potentiated by neuromuscular blocking agents. This myopathy differs from the chronic myopathy caused by long term corticotherapy. After a status asthmaticus treated with corticosteroids and mechanical ventilation under muscle relaxants, creatine kinases should be measured in plasma and a neurological examination be performed. The value of neuromuscular blockade monitoring for the prevention of this myopathy should be assessed.