Data on incidence, prevalence and mortality of idiopathic pulmonary fibrosis (IPF) are sparse and vary across studies. The true incidence and prevalence of the disease are unknown. In general, the overall prevalence and incidence reported in European and Asian countries are lower than those reported in American studies. In recent years, the epidemiological approach to IPF has been difficult for many reasons. First, the diagnostic criteria of the disease have changed over time. Secondly, the coding system used for IPF in administrative databases, the most common data source used to study this aspect of the disease, has been modified in the past few years. Finally, the study design, the methodology and the population selected in each of the studies are very different. All these aspects make comparisons among studies very difficult or impossible. In this review, we list the main issues that might arise when comparing different studies and that should be taken into consideration when describing the state of epidemiological knowledge concerning this pathology.