Whilst the acute effects of sulfur mustard have been relatively well characterised, the chronic effects of short term but significant exposures are still evolving. The approximately 30,000 Iranian victims of CW exposure from the 1980 to 1988 Iran-Iraq war who are currently being followed form a key population who are now 20 years post-exposure. The key chronic findings in this population reflect the common acute effects of sulfur mustard, and are related to the skin, eye and respiratory system. Excluding pruritus, skin changes appear to settle. Eye symptoms are slowly progressive, however a severe, rapid onset form of keratitis is seen to develop in a number of patients after a latent period of 15-20 years. The respiratory tract also shows progressive deterioration, with bronchiolitis obliterans now being considered the main pathological feature of "mustard lung". In addition, there are other potential effects of sulfur mustard exposure which become evident only in the longer term and which are being investigated, including the development of cancer, immunological and neuropsychiatric changes, and reproductive effects. Finally, a chronic effect of sulfur mustard exposure that is now becoming apparent is the wider long-term social and economic effects of these illnesses on individuals and their families.