Pefloxacine 800 mg single dose was given as routine treatment after a cystomanometric examination to a 45-year-old woman with a 30-month history of generalized myasthenia gravis. One hour after, the patient developed exacerbation of myasthenia gravis with bilateral ptosis and an increased generalized weakness. She experienced a rapid improvement during the next 8 hours and physical examination returned to normal within one day. No additional factors which might have contributed to the exacerbation of myasthenia gravis were found. The report of exacerbation of myasthenia gravis with other antibiotic belonging to the group of fluoroquinolones (ciprofloxacin, norfloxacin and ofloxacin) prompt us to recommend caution with the use of all fluoroquinolones in myasthenic patients.