BACKGROUND: Myasthenia gravis is an autoimmune neuromuscular disorder that shows increased sensitivity to nondepolarizing muscle relaxants. Mivacurium chloride is a short acting nondepolarizing neuromuscular blocker and a benzylisoquinolin diester that is hydrolized rapidly to inactive metabolites by plasma pseudocholinesterase. The onset and duration of mivacurium in patients undergoing a thymectomy with myasthenia gravis was studied. METHODS: Fifteen patients undergoing a thymectomy for myasthenia gravis and fifteen patients of ASA class I, II without liver, kidney or neuromuscular disease undergoing orthopedic surgical procedures were included in this study. Anesthesia was induced with thiopental 4 5 mg/kg and maintained with inhalation of N2O:O2 (1:1) and enflurane 1.0 2.0 vol%. Mivacurium 0.2 mg/kg was given as a muscle relaxant and then intubation performed after the twitch response was depressed more than 90%. Neuromuscular relaxation was assessed by TOF (T1) at the adductor pollicis with supramaximal stimulation of the ulnar nerve at 2 Hz every 12 seconds. The onset and duration of 5%, 25%, 50%, 75%, and 95% recovery time of T1 and recovery index were recorded. RESULTS: Onset of block was shortened but recovery time of 5%, 25%, 50%, 75%, 95% and recovery index were prolonged in patients with myasthenia gravis. CONCLUSIONS: In patients with myasthenia gravis, mivacurium induced rapid onset time and prolonged recovery time of 5%, 25%, 50%, 75%, 95% and recovery index.