The authors describe a clinical case of occupational asthma due to dioctyl-phthalate in a worker using a conveyor belt for bottle stoppers. The dioctyl-phthalate is used for the production of PVC and as a compound of many other substances. The literature describes many cases of occupational asthma do to phthalate anhydride but few cases do to dioctyl phthalate. The clinical history, the environmental monitoring data, the pulmonary function tests such as bronchodilatation test by salbutamol, peak flow meter daily monitoring for three weeks, and stop and start work test showed a good correlation with the work and the worker's asthma. The authors emphasize that a detailed clinical history and a careful analysis of the production cycle are very useful and, as simple and non invasive tests, can play an important role in the diagnosis of occupational asthma.