Several sources of evidence, including investigations of pathogenesis and observational studies, support the hypothesis that environmental agents may have an etiologic role in idiopathic pulmonary fibrosis (IPF). Since 1990, six case-control studies have been conducted in three countries and have consistently demonstrated increased risk of IPF with exposures to a number of environmental and occupational agents. In a meta-analysis of these studies, six exposures were significantly associated with IPF (summary odds ratios [95% confidence intervals]), including ever smoking (1.58 [1.27-1.97]), agriculture/farming (1.65 [1.20-2.26]), livestock (2.17 [1.28-3.68]), wood dust (1.94 [1.34-2.81]), metal dust (2.44 [1.74-3.40]), and stone/sand (1.97 [1.09-3.55]). Although there are a number of limitations of the case-control design and these results alone do not establish a causal link, an assessment of all of the available evidence strongly suggests that IPF may be a heterogeneous disorder caused by a number of environmental and occupational exposures.