Caregiver distress is a complex, multicausal phenomenon. Most of the available interventions for caregivers are protocolized or manualized. A case formulation approach may be more appropriate, as it provides the opportunity for assessing and treating specific dysfunctional mechanisms explaining each caregiver's needs and causes of distress, through the design of an individualized modular intervention. In the present study, a randomized controlled study has been done in which a functional analysis-guided modular intervention (FAMI) was developed. The FAMI was compared with two manualized intervention (cognitive-behavioral therapy and acceptance and commitment therapy) which have received preliminary support, and a control group (waiting list). Mixed-effect models and clinical significance analyses were performed. Results suggest that all the interventions were effective in reducing depressive and anxious symptomatology at postintervention assessment, with large effect sizes, and with reductions in depression being maintained at follow-up. Regarding anxiety, long-term effects have been found only for the FAMI intervention. Results also suggest positive effects associated with the FAMI intervention in the long term with regards to comorbidity between depressive and anxious symptomatology. FAMI presents great potential for reducing dementia caregivers' levels of distress, being especially effective in maintaining therapeutic effects in the long term. (PsycINFO Database Record (c) 2020 APA, all rights reserved).