The purpose of this study was to investigate whether preventive measures such as reduction of exposure and the introduction of the histamine provocation test (HPT) as a selection instrument resulted in a lower incidence of potroom asthma (PA) and a longer time lag between the commencement of employment and the occurrence of PA. Between 1970 and 1990, 179 cases of PA were diagnosed. This period was divided into three periods. During period 1 (1970-1975), no exposure data were available. Period 2 (1976-1981) is characterized by known exposure data obtained by means of fluoride determinations in urine. At the beginning of period 3 (1982-1990) the HPT was incorporated into the preemployment medical examination. We computed the incidence density (ID) in the three periods and analyzed the timelag in relation to the year of employment and confounding factors such as age, atopic history, blood eosinophil counts, lung function, smoking habits at preemployment, and exposure level. After introduction of the preemployment HPT the ID decreased, but cases continued to occur (ID 11.6 in period 2 versus 2.5 in period 3). The time lag was did not differ when subjects with bronchial hyperresponsiveness were screened out. The exposure level and an atopic history were factors associated with the period of employment and, therefore, confounded the results. The results of this study support the role of an atopic history as a risk factor for development of PA at lower exposure levels and suggest that potroom exposure not only incites asthmatic symptoms but also acts as an inducer of respiratory disease.