Phthalic anhydride (PA) is well known as a chemical substance which is a potent irritant of mocous membranes and a possible cause of pulmonary sensitization. In a previous study, the authors reported a case of health disorder due to PA in a male worker engaged in the polymerization process of alkyd resin. He had been exposed to high concentrations of PA while throwing PA powder into the reactor and cleaning the floor, and had complained of symptoms such as sneezing, coughing, sore throat, urticarial exanthema and conjunctival hyperemia, which gradually appeared and culminated in a dyspneic attack 3 years after he began working in the factory. Provocation test suggested a diagnosis of bronchial asthma due to PA. After the occurrence of this case, a closed system using molten PA was introduced into the plant in 1968. In this paper, the results of a comparative study made in 1976 on the health conditions of PA-workers with control groups such as melamine-polyester workers, paint-producing workers and office workers, as well as measurements of PA concentrations in the ambient air under the present working conditions are reported. In that study questionnaires inquiring about occupational history and subjective symptoms were given and spirometries were also performed. PA concentrations in the air of the working area were measured using an ultraviolet spectro-photometer. The results were as follows: 1) In 1971, the rates of respiratory and eye symptoms among the PA-workers who had engaged in PA work before the introduction of the closed system were higher than the rates among those who started working after the introduction of the system. 2) In 1976, the rates of subjective symptoms among those who had engaged in PA work before 1968 became similar to the rates of those who started working after 1968. 3) The rates of respiratory and other subjective symptoms among PA-workers in 1971 were higher than those among the three control groups and also higher than those of PA-workers in 1976. 4) The rate sof subjective symptoms among PA-workers were almost the same as the melaminepolyester and paint-producing workers in 1976. 5) There were no statistical differences in pulmonary functions (%VC, %FEV_1.0″, MVV/BSA, PFR, V₂₅, V₅₀, V₂₅/Ht, V₅₀/Ht, V₅₀/V₂₅ and V₅₀/BSA) observed between PA-workers and other control groups, except V₅₀/V₂₅ between the paint-producing workers and office workers. 6) PA concentrations in the air ranged from undetectable amount up to 1.1 ppm under the closed system. Concentrations in the air were measured to be 24.9 ppm during the powder-throwing procedure and 77.3 ppm in areas where workers engaged in floor-cleaning before the introduction of the closed system. PA concentrations in the air escaping from the storage tank were 28.8 ppm, whlie concentrations in the air from the scaling tank ranged from 73.5 to 80.5 ppm. A recovery apparatus was subsequently attached to the end of the scaling tank vent pipe to prevent the escaping gas from contaminating the atmosphere. This procedure was effective in reducing PA concentrations in the air to 0.3-3.0 ppm. The authors conclude that the closed system was effective in improving health conditions among PA-workers resulting from the improvement of PA-working conditions.