Four of 133 patients with rheumatoid arthritis suddenly developed progressive respiratory symptoms while receiving penicillamine therapy. None had previous disease. None of 89 patients treated with gold alone and 0 of 380 not taking gold or penicillamine developed respiratory symptoms. Two of the 4 developed simultaneous severe, unresponsive sinusitis. Lung biopsy demonstrated obliterative bronchiolitis (OB) in 2 patients, fibrosing alveolitis in one. A clinical course characteristic of OB was noted in the 4th patient. Sinus biopsy in 2 demonstrated lymphocytic and plasmacytic mucosal infiltration. The high frequency of OB in penicillamine treated patients and its absence in 469 non penicillamine treated patients suggests the possibility of a causal relationship.