Many patients with sulfur mustard (SM) exposure present dyspnea in exertion while they have a normal pulmonary function test (PFT) and imaging. The cardiopulmonary exercise test (CPET) has been used for evaluation of dyspnea in exertion among patients with different pulmonary disorders focusing on assessing gas exchange. We evaluated subjects who were exposed to SM with normal imaging compared to the controls with CPET. A case-control study was carried out on two groups in Tehran, Iran during 2010 to compare the CPET findings. The cases with a history of SM exposure and complaint of exertional dyspnea while they had normal physical examination, chest X-ray, PFT, and nonsignificant air trapping in lung high resolution computed tomography (HRCT) were included. A group of sex- and age-matched healthy people were considered as controls. One hundred fifty-nine male patients (aged 37 ± 4.3 years) were enrolled as a case group and ten healthy subjects (aged 35 ± 5.9 years) as the control group. There was no significant difference in the demographic and baseline PFT characters between the two groups (P> 0.05). Only peak VO2/kg, VO2-predicted, and RR peak were statistically different between cases and controls (P < 0.05). Despite the fact that abnormal gas exchange may be present in our cases, it does not explain the low VO2 in CPET. Also, impaired cell O2 consumption could be a hypothesis for low VO2 in these cases. It seems that routine assessment of lung structure cannot be effectively used for discrimination of the etiology of dyspnea in low-dose SM exposed cases.