D-penicillamine, a drug used to treat rheumatoid arthritis, Wilson's disease, and cystinuria, can cause myasthenia gravis. Fortunately, the myasthenia typically resolves after discontinuation of the drug. The diagnosis may be missed if weakness is blamed on a patient's underlying disease(s), in particular, rheumatoid arthritis. Reported here are the cases of two patients with chronic obstructive lung disease who were taking D-penicillamine for rheumatoid arthritis, then experienced increasing respiratory failure. At first, their problem seemed to stem from chronic lung disease, but further evaluation revealed the cause of the hypoventilation to be D-penicillamine-induced myasthenia gravis.