ImportanceOutreach messages to patients overdue for well child care (WCC) can be delivered different ways (ie, telephone calls and text messages). Use of electronic health record patient portals is increasingly common but their effectiveness is uncertain.ObjectiveTo determine the effectiveness of patient portal outreach messages, with and without the date of the last WCC, on the scheduling and completion of WCC visits and completion of vaccinations.Design, Setting, and ParticipantsAn intention-to-treat, multigroup, randomized clinical trial was conducted at 3 academic primary care practices from July 30 to October 4, 2021. The population included predominantly non-Hispanic Black, low-income children (age, 6-17 years) whose parent had an active portal account.InterventionsParticipants were randomized to the standard message, tailored message, or no message (control) group. Two messages were delivered to those in the message groups.Main Outcomes and MeasuresOutcomes included WCC visit scheduled within 2 weeks of the first intervention message, WCC visit completed within 8 weeks (primary outcome), and receipt of COVID-19 vaccine within 8 weeks.ResultsNine hundred forty-five patients participated (mean [SD] age, 9.9 [3.3] years, 493 [52.2%] girls, 590 [62.4%] non-Hispanic Black, 807 [85.4%] publicly insured). Scheduling rates were 18.4% in the standard message group (adjusted risk ratio [aRR], 1.97; 95% CI, 1.32-2.84) and 14.9% in the tailored message group (aRR, 1.57; 95% CI, 1.02-2.34) compared with the control group (9.5%). Well child care visit completion rates were 24.1% in the standard message group (aRR, 1.92; 95% CI, 1.38-2.60) and 19.4% in the tailored message group (aRR, 1.52; 95% CI, 1.06-2.13) compared with the control group (12.7%). Among eligible children, rates of receiving the COVID-19 vaccine were 16.7% in the standard message group compared with 4.8% in the tailored message (aRR, 3.41; 95% CI, 1.14-9.58) and 3.7% in the control groups (aRR, 4.84; 95% CI, 1.44-15.12).Conclusions and RelevanceIn this randomized clinical trial, outreach messages delivered via electronic health record patient portals increased the rates of scheduling and completing WCC visits and receiving the COVID-19 vaccine, providing a useful tool to help restore WCC in populations whose care was delayed during the pandemic.Trial RegistrationClinicalTrials.gov Identifier: NCT04994691