A case is reported of prolonged neuromuscular block after mivacurium chloride for laparoscopic cholecystectomy, in a 45 years old patient, treated with cyclophosphamide for a Wegener's granulomatosis. The neuromuscular function monitoring by train-of-four showed a duration of action of 75 min after an intubation dose of 0.20 mg.kg-1. Additional bolus of 1 mg, corresponding to 25% of usual doses, every 10 or 15 min, were sufficient for maintaining muscle relaxation. Spontaneous recovery, without any antagonization, lasted 40 min for a TOF ratio (T4/T1) > or = 70%. Recovery index from 25 to 75% were 13 min. Plasma butyrilcholinesterases activity were reduced to a level of 50%. With reference to literature about succinylcholine, the responsibility of cyclophosphamide is likely, and discussed. This observation shows the value of monitoring the neuromuscular transmission.