Occupational exposures were investigated in a multicenter case-control study of clinically and histologically diagnosed idiopathic pulmonary fibrosis (IPF), a chronic diffuse interstitial lung disease of unknown etiology. Results are based on 248 cases, aged 20–75 years, diagnosed at 16 referral centers between January 1989 and July 1993. There were 491 controls ascertained by random digit dialing and matched to cases on sex, age, and geographic region. Data were collected using a standard telephone questionnaire. Occupational factors were based on a detailed history of jobs lasting 6 months or more and job activity, hobby, and specific substance checklists. Several occupational factors, adjusted for age and smoking in conditional multivariate logistic regression analyses, were significantly associated with IPF: farming (odds ratio (OR) = 1.6, 95% confidence interval (CI): 1.0, 2.5); livestock (OR = 2.7, 95% CI: 1.3, 5.5); hairdressing (OR = 4.4, 95% CI: 1.2, 16.3); metal dust (OR = 2.0, 95% CI: 1.0, 4.0); raising birds (OR = 4.7, 95% CI: 1.6, 14.1); stone cutting/polishing (OR = 3.9, 95% CI: 1.2, 12.7); and vegetable dust/animal dust (OR = 4.7, 95% CI: 2.1, 10.4). Interaction was detected between smoking and exposure to livestock (p = 0.06) and farming (p = 0.08). Results confirm previous studies showing increased risk associated with dusty environments. Am J Epidemiol 2000;152:307–15.