A middle-age woman had intermittently had chloroquine as an antimalarial agent for 21 years. Although she had discontinued the drug for more than 10 years due to occurrence of a retinopathy, mild ocular myasthenic symptoms persisted. Cardiac conduction disturbances were detected as well. A quadriceps muscle biopsy revealed mild neurogenic changes and interstitial lymphorrhages. The decremental response after repetitive stimulation was reversed by edrophonium administration. The history suggests that the persisting myasthenia might have been caused by chloroquine intake.