BACKGROUND: The adverse health effects of welding come from chemical, physical, and radiation hazards. An outbreak of bronchitis was reported among welders at a manufacture factory in Pohang at Sep 1996. The object of this study is to assess the cause of the bronchitis in the welders. METHODS: Authors conducted a questionnaire survey for 12 welders in the factory and a family physician examined their health status. Authors also collected their routine health examination reports and air concentrations of their work environment at that time. RESULTS: The incidence rate of the bronchitis among welders was 91.7%(11 cases among 12 welders). The symptoms were developed at Feb 1996, 3 months after welding against steelplates. They were aggravated during worktime and relieved at weekend. The symptoms were cough(100.0% ), sputum(90.9% ), sore throat(72.7% ), and general fatigue(72.7% ). No abnormal findings were observed in the physical examinations and the radiologic findings. Though air concentrations of welding fume in the confined space(geometric mean = 14.68mg/m3) were over the permissible exposure limit(5mg/m3), those in open space(geometric mean=3.66mg/m3) were below. The air concentrations of metal components(Mn, Pb, Zn, Cu, Fe, Ni) in the fume were within each of the permissible exposure limit. CONCLUSIONS: Authors thought that the outbreak of bronchitis was brought about by CO2 arc welding against paint-free steelplates. Further studies on the factors contributing to the bronchitis will be needed.