This study investigated the relationship between two therapist attributes (reflective functioning and attachment style) and client outcome. Twenty-five therapists treated a total of 1001 clients. Therapists were assessed for reflective functioning and attachment style using the Adult Attachment Interview and the Experiences in Close Relationships Scale. Clinical outcome was measured using the Outcome Questionnaire (OQ-45). Data were analysed using hierarchical linear modelling. Results indicated that therapist reflective functioning predicted therapist effectiveness, whereas attachment style did not. However, there was evidence of an interaction between therapist attachment style and therapist reflective functioning. Secure attachment compensated somewhat for low reflective functioning and high reflective functioning compensated for insecure attachment. Possible implications for the selection of therapy training candidates and therapist training are discussed.