Mustard gas or sulfur mustard (SM) is an alkylating chemical warfare agent that was widely used during the World War I and in the Iran-Iraq conflict. We aimed to study late toxic effects of SM on the respiratory system of severely intoxicated Iranian veterans. Respiratory examination, spirometry, arterial blood gas (ABG) analysis, and high resolution computed tomograpghy (HRCT) of the chest were performed on all severely SM-poisoned veterans in the province of Khorasan, Iran. HRCT abnormalities were classified into four grades based on the number of lung lobes involved. ABG and spirometric results were compared with each other, as well as, with the severity grades of HRCT abnormalities, using Spearman's rank correlation test. Forty male subjects with confirmed SM poisoning 16 to 20 years ago, were studied. Main respiratory complications were diagnosed as chronic obstructive pulmonary disease (COPD) (35%), bronchiectasis (32.5%), asthma (25%), large airway narrowing (15%), pulmonary fibrosis (7.5%), and simple chronic bronchitis (5%) patients. While there was a significant correlation (p<0.05) between ABG and spirometric results, the severity grades of HRCT abnormalities revealed a significant correlation (p<0.05) only with PaO2. We concluded that SM-induced respiratory complications tend to progress over the years. While spirometry is a valuable diagnostic tool for evaluation of pulmonary impairment during regular follow-ups, ABG and HRCT are more objective and should be more considered for evaluation of the severity and for diagnosis of the respiratory complications.