Therapist adherence describes the quality of interventions according to the imperatives of a treatment model. We examined the relationship between therapist adherence and symptom change in the context of a short-term treatment with respect good psychiatric management (GPM) principles. Based on a parent trial, borderline personality disorder patients (N = 40) benefited from a 10-session intervention. Adherence to GPM was assessed using a GPM Adherence Scale (GPMAS). The psychometric properties of the GPMAS were excellent, and the adherence to GPM explained 16% of the general symptom improvement (t(1) = 2.38, β = 0.40, p = 0.02) and 23% of the borderline symptom improvement (t(1) = 2.46, β = 0.48, p = 0.02). Because GPM adherence predicts the outcome after only 10 sessions, GPMAS is a valuable measure early on in psychiatric practice as part of an initial step to longer-term treatment, to quickly detect problems and correct them.