To investigate early pulmonary responses to metalworking fluid exposure, we enrolled first-year machinist apprentices and apprentices in three other trades into a 2-yr longitudinal study. We obtained complete data for 82 machinists and 159 control subjects. Tests included respiratory questionnaires, spirometry, methacholine challenge, and allergy skin tests. Details on duration of exposure were collected by interview and 68 representative full shift personal samples for "total aerosol" were obtained from 13 shops (mean: 0.46 mg/m3, range: < 0.7 to 3.65 mg/m3). Machinists and control subjects did not differ at baseline. At follow-up, average change in bronchial responsiveness was double in machinists compared with control subjects (p = 0.05), and machinists were more likely to have developed new bronchial hyperresponsiveness (BHR) with asthmalike symptoms. In linear regression analysis, for predictors of methacholine slope, increased BHR was associated with duration of exposure to both synthetic and soluble metalworking fluids (p < 0.05); in logistic regression analysis, for predictors of BHR, only duration of exposure to synthetic fluids was a significant predictor. Results were not changed when workers with PC20 < 8 mg/ml at baseline were excluded. We conclude that exposure to water-based metalworking fluids (especially synthetic fluids) is associated with increasing BHR during the first 2 yr of exposure.