Chiapa, AmandaSmith, Justin D.Kim, Han JoeDishion, Thomas J.Shaw, Daniel S.Wilson, Melvin N.2020-03-102020-03-1010/1/2015Copyright 2015 Journal of Consulting and Clinical Psychology. This is a post-print version of a published paper that is available at: https://psycnet.apa.org/record/2015-28936-001 Recommended citation: Chiapa, Amanda, Justin D. Smith, Hanjoe Kim, Thomas J. Dishion, Daniel S. Shaw, and Melvin N. Wilson. "The Trajectory of Fidelity in a Multiyear Trial of the Family Check-Up Predicts Change in Child Problem Behavior." Journal of consulting and clinical psychology 83, no. 5 (2015): 1006. DOI: 10.1037/ccp0000034 This item has been deposited in accordance with publisher copyright and licensing terms and with the author’s permission.https://hdl.handle.net/10657/6161Therapist fidelity to evidence-based family interventions has consistently been linked to child and family outcomes. However, few studies evaluate the potential ebb and flow of fidelity of therapists over time. We examined therapist drift in fidelity over four years in the context of a Family Check-Up prevention services in early childhood (age 2–5). At age 2, families engaging in Women, Infants, and Children Nutritional Supplement Program (WIC) services were randomized and offered annual Family Check-Ups. Seventy-nine families with a child in the clinical range of problem behaviors at age 2 were included in this analysis. Latent growth modeling revealed a significant linear decline in fidelity over time (M = ?0.35, SD = 0.35) and steeper declines were related to less improvement in caregiver-reported problem behaviors assessed at ages 7.5/8.5 (b = ?.69, p = .003; ? = ?.95, CI: ?2.11 | ?0.22). These findings add to the literature concerning the need to continually monitor therapist fidelity to an evidence-based practice over time to optimize family benefits. Limitations and directions for future research are discussed.en-USearly childhoodexternalizingFamily Check-Upfidelityimplementationrandomized controlled trialarticle