Background: Therapist self-practice, in its various forms, is common across therapeutic modalities, but a measure of its impact on participants does not yet exist. Aims: This paper describes the development and reliability testing of the Self-focused Practice Questionnaire' (SfPQ), a measure of self-perceived impact of one's self-focused practice. Method: Exploratory factor analysis (EFA), internal consistency and reliability were assessed in a convenience sample of 112 trainee therapists. Results: Five factors, rating impacts on therapist Personal-self (Awareness of Developmental experiences, Experience of Personal Change and Felt-sense/Self-awareness), and Therapist-self (Internalization of the Model, and Development of Empathy) were identified, with good internal consistency and acceptable to good test-retest reliability. Conclusions: Though more work is needed, these preliminary results support the SfPQ's reliability and validity. The SfPQ is an important measure, which may enhance routine rating of self-focused practice in training institutions.