The case formulation and cognitive-behavior therapy of a patient whose pathology fulfills criteria for both obsessive-compulsive (OCD) and paranoid personality disorders (PPD) is described. Case formulation encompasses all the relevant cognitive, emotional and motivational factors underlying he disorders, emphasizing interactions between processes and strategies aimed at investigating and disconfirming feared scenarios, and reconstructing the ontogeny of pathological beliefs and themes n the patient’s life history. The treatment targeted rumination and dysfunctional interpersonal cycles hat maintained and reinforced pathology, promoting awareness, critical insights, risk acceptance and
progressive decline of maladaptive control strategies. The challenges to the therapeutic alliance due to paranoid ideation towards the therapist and the relational strategies used to overcome them are also reported. The case offers clear indications about effective interventions that address both common and specific mechanisms involved in OCD and PPD, and provides observations that might foster future theoretical conceptualizations and research on the involvement of guilt, shame and humiliation in the two conditions.