Interstitial lung disease was diagnosed in a 52-yr-old male diamond polisher, who worked with polishing disks containing cobalt. After a further 7 months of probably high occupational exposure without any specific treatment, he had to quit work because of dyspnea. Despite treatment with systemic corticosteroids and continuous oxygen administration, he died 3 months later in respiratory distress. Postmortem examination of the lung tissue showed a typical giant-cell interstitial fibrosis, with active inflammatory cell infiltration superimposed on an established centrilobular fibrosis. The lung tissue contained 2.1 micrograms cobalt/g wet weight (more than 100-fold the normal concentration); cobalt particles, mainly localized in macrophages, were identified by transmission electron microscopy and energy-dispersive X-ray analysis. We speculate that the rapid deterioration and fatal outcome resulted from the continued exposure to cobalt, leading to a high pulmonary concentration of cobalt, and from the oxygen treatment because cobalt promotes the formation of hydroxyl free radicals.