Mustard gas (HD) was a widely used chemical warfare agent during World War I and more recently in the Iraq-Iran war (1980-1988). To date, dramatically, 45,000 Iranians are suffering from late respiratory complications due to MG exposure. This review covers two decades of researches on latent pulmonary effects of MG. Findings from clinical manifestations, pathologic examinations, laboratory data, lung function tests, and radiological evaluations are reviewed. From this review we are able to provide a suitable practical plan for workup and management of patients in this setting.