The Accrual to Clinical Trials (ACT) Ontology supports queries across multiple research institutions in the ACT Network, a nationwide federation of leading academic research institutions that share aggregate patient counts from electronic health record (EHR) data. Its development is funded by the NIH through the National Center for Advancing Translational Sciences (NCATS) and the Clinical and Translational Science Award (CTSA) program. It contains over 125 million patient records. Usability of the ACT ontology will improve the precision, recall, and reproducibility of identifying clinical cohorts in EHR data sets.