Epidemiological studies have shown that exposure to welding fumes can be a cause of occupational asthma (OA), although the mechanisms implicated are unknown. We describe 3 patients (all men, mean age 42 years) with OA secondary to exposure to welding fumes generated during metal arc welding on iron. The exposure time ranged from 7 to 43 years and the time of the onset of symptoms following the start of exposure was 2–12 years. Patients were diagnosed by specific inhalation challenge (SIC). Environmental levels of Fe, Cd, Cu, Cr, Ni, NO₂, NO, CO, and O₃ produced during the SIC did not exceed threshold limit values. Samples of induced sputum were obtained before and after the SIC and showed an increase in neutrophils and concentrations of IL-8, TNF-α and TNF-β after the SIC. This study presents the first clinical findings reported in welders with OA, mainly working with iron. Neutrophilic inflammation seems to play a role in this disease.