Precision medicine is an approach that combines individual clinical characteristics with additional biological information to distinguish between patients with similar diagnoses, with the aim of predicting disease course and treatment response. Biomarkers have an important role in the practical implementation of precision medicine and enrichment of clinical trial populations. For patients with chronic obstructive pulmonary disease, higher blood eosinophil counts, in combination with clinical characteristics, are associated with increased response to corticosteroids and targeted therapies. A single blood eosinophil count cutoff value is not appropriate for identification of potential treatment response, because often a continuous relationship exists between blood eosinophil counts and treatment response. Response also varies with clinical characteristics such as exacerbation risk and current smoking status. Future incorporation of blood eosinophil counts into clinical trials could potentially reduce drug development costs and trial failures. Including blood eosinophil counts as a biomarker in clinical trials will also provide further opportunities to evaluate aspects of eosinophil testing and standardization of methodology and clinical utility. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (