Five patients out of 71 with rheumatoid arthritis (RA), who received D-penicillamine, developed myasthenia gravis (MG) within a two-year period. They all responded promptly to discontinuation of the drug and pyridostigmine administration. None of the patients had anti-Ro(SSA) antibodies or features of Sjögren's syndrome, whereas three of the five had the HLA-DR1 phenotype. The relatively high frequency of MG observed in our population, along with its unpredictability and potentially serious sequelae, necessitates its inclusion in the list of side effects of D-penicillamine routinely discussed with the patient, prior to initiation of the treatment. Full alertness of both the patient and the physician to even minor initial myasthenic symptoms, that dictate immediate discontinuation of the drug, is of obvious importance.