We have used mivacurium in two myasthenic patients, a generalized myasthenia gravis (MG) patient presenting for thymectomy and a Lambert-Eaton myasthenic (LEM) patient for mediastinoscopic lymph node biopsy. Both of them received nitrous oxide/oxygen (1:1)-narcotic-enflurane anesthesia with mivacurium as a muscle relaxant and the neuromuscular blocking effect of mivacurium was monitored continuously through the operation as well as before the induction of anesthesia. The dose of mivacurium for MG patient was 5.5 mg and LEM patient was 12 mg, because MG patient showed more severe clinical symptoms. The response to train-of-four (TOF) ulnar nerve stimulation was recorded using accelography. The onset times to maximal block in MG and LEM patients were 30 and 120 sec, respectively after injection and the recovery times to 25% from maximal block were 117 and 76 min, respectively. Mivacrium would be safe and appropriate for use in myasthenic patients, with relatively small dose under the neuromuscular monitoring.