Background/aim:Cost-efficient methods are essential for successful participant recruitment in clinical trials. Patient portal messages are an emerging means of recruiting potentially eligible patients into trials. We assessed the response rate and complaint rate from direct-to-patient, targeted recruitment through patient portals of an electronic medical record for a clinical trial, and compared response rates by differences in message content.Methods:The Study to Understand Fall Reduction and Vitamin D in You (STURDY) trial is a National Institutes of Health–sponsored, community-based study of vitamin D supplementation for fall prevention in older adults conducted at Johns Hopkins. Potential participants were identified using the Epic electronic medical record at the Johns Hopkins Health System based on age (≥70 years), ZIP code (30-mile radius of study site), and prior activation of a patient portal account. We prepared a shorter message and a longer message. Both had basic information about study participation, but the longer message also contained information about the significance of the study and a personal invitation from the STURDY principal investigator. The Hopkins Institutional Review Board did not require prior consent from the patient or their providers. We calculated the response rate and tracked the number of complaints and requests for removal from future messages. We also determined response rate according to message content.Results:Of the 5.5 million individuals receiving care at the Johns Hopkins Health System, a sample of 6896 met our inclusion criteria and were sent one patient portal recruitment message between 6 April 2017 and 3 August 2017. Assessment of enrollment by this method ended on 1 December 2017. There were 116 patients who expressed interest in the study (response rate: 1.7%). Twelve (0.2%) recipients were randomized. There were two complaints (0.03%) and one request to unsubscribe from future recruitment messages (0.01%). Response rate was higher with the longer message than the shorter message (2.1% vs 1.2%; p = 0.005).Conclusion:Patient portal messages inviting seniors to participate in a randomized controlled trial resulted in a response rate similar to commercial email marketing and resulted in very few complaints or opt-out requests. Furthermore, a longer message with more content enhanced response rate. Recruitment through patient portals might be an effective strategy to enroll trial participants.