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Introduction: Achieving adequate retention rates in clinical trials is essential to ensuring meaningful results. Although financial reimbursement is an effective strategy to increase participant retention, current policies restrict the use of federal funds to reimburse U.S. active duty Service members for research participation. It is unknown whether permitting financial reimbursement among this population would improve trial retention rates. A recent randomized effectiveness trial received approval to provide reimbursement to Service member participants several months after recruitment began, creating a natural experiment to study the effects of financial reimbursement on retention. Materials and methods: Active duty Service members recruited from six U.S. military treatment facilities (N = 666) were enrolled in a collaborative care study and completed assessments at baseline, three-, six-, and 12-months. Data on study assessment completion rates at three- and six-months were analyzed using the mixed-effects binary logit model to determine the probabilities of completing assessments based on reimbursement status. Results: Participants who received reimbursement were significantly more likely to complete study assessments at both time-points than participants who did not receive reimbursement (p