Rare earth pneumoconiosis is an uncommonly reported disease caused by the inhalation of dust containing lanthanides, also known as rare earth metals, which are common industrial materials. The pathologic manifestations and natural history of this disorder are incompletely understood. We describe a male patient with a 35-year history of optical lens grinding, an occupation associated with exposure to cerium oxide, a rare earth metal-containing compound. The patient presented with progressive dyspnea and an interstitial pattern on chest X-ray; open lung biopsy showed interstitial fibrosis histologically indistinguishable from usual interstitial pneumonitis. However, scanning electron microscopy with energy-dispersive X-ray analysis demonstrated numerous particulate deposits in the lung, of which the majority contained the rare earth metal cerium alone or in combination with other elements. Our case is one of the first to describe rare earth pneumoconiosis associated with pulmonary fibrosis in the occupational setting of optical lens manufacture. Besides reinforcing the contention that rare earth metals are potentially harmful, our case suggests that such agents may be causally related to the development of pulmonary fibrosis.