Background: Family members who care for patients with severe mental illness experience emotional distress and report a higher incidence of mental illness than those in the general population. They report feeling inadequately prepared to provide the necessary practical and emotional support for these patients. The MAT training, an Interaction-Skills Training program (IST) for caregivers, was developed to meet those needs. This study used a single-arm pretest-posttest design to examine the impact of the training on caregivers' sense of competence (self-efficacy) and burden. Methods: One hundred family caregivers recruited from three mental health institutions participated in the training. Burden was assessed using the Involvement Evaluation Questionnaire, and self-efficacy using the Self-Efficacy Questionnaire. Analysis of variance with repeated measures was used to investigate whether participation in the training changed the level of family caregivers' burden and self-efficacy. Pearson's correlation was used to examine the relationships between self-efficacy and burden. Results: Our results indicate that, after the training, self-efficacy increased significantly over time (p < 0.001) and that burden decreased significantly (p < 0.001). However, the results could not demonstrate the expected association between an increase of self-efficacy and decrease of burden. Caregivers expressed high appreciation for the training. Conclusions: After following the IST program, family caregivers of patients with severe mental illness experienced a greater sense of competence and a significant decrease in burden. The training was greatly appreciated and satisfied caregivers' need to acquire the skills required in complex caregiving situations.