Hard metal lung disease is a rare form of occupational lung disease that can occur in workers engaged in the manufacture, utilization, or maintenance of tools composed of hard metal [a material composed mainly of tungsten carbide (WC) and cobalt] or diamond-cobalt. Clinically, the condition resembles hypersensitivity pneumonitis, with subacute presentations and possible evolution to pulmonary fibrosis. However, this interstitial lung disease is uniquely characterized by the presence of bizarre ;;cannibalistic'' multinucleated giant cells in the alveoli and the bronchoalveolar lavage. A pathological diagnosis of giant cell interstitial pneumonitis (GIP) is, therefore, specific for hard metal lung disease, even though not all affected subjects exhibit this pathognomonic feature. Cobalt is the critical toxic component causing hard metal lung disease, hence also the term cobalt-lung. Hard metal lung disease is more likely to occur in poorly regulated workplaces, but its occurrence depends mainly on individual susceptibility, rather than on cumulative exposure, so that even young subjects may be affected.