This research aimed to establish the effect of Functional Analytic Psychotherapy (FAP) on the clinically relevant behaviors of adults with emotional regulation difficulties attending counseling center. An AB design was raised with replication in three subjects and was established as a unit of measure the absolute frequency of clinically relevant behaviors type 1 (CRB1) and Clinically Relevant Behaviors type 2 (CRB2) as well as problematic behaviors out of session by using structured and anecdotal registers. The results indicate criteria of effectiveness and efficacy of the intervention in two of the cases discussed. Therapeutic Implications are discussed, considering problematic behaviors in and out of session, the use of rules by the therapist and the relationship between them.