Medication non-adherence is a major obstacle in the treatment of affective disorders. The primary objective of this study was to evaluate two different interventions to improve adherence to antidepressant drugs. Secondary objectives included response to treatment, relation between adherence and response, patient satisfaction and tolerability. A randomized controlled design was used to assess the effect of a patient educational compliance enhancing programme (CP) and therapeutic drug monitoring in 1031 major depressed patients treated with sertraline for 24 weeks and managed by their general practitioner. Adherence was measured by questioning, measurable serum levels of sertraline and desmethylsertraline, appointments kept and a composite index including all three methods. Treatment adherence was found in 37-70% of patients, depending on the method used. Neither of the interventions resulted in a significant increase in adherence rate. However, significantly more patients in the CP group had responded at week 24 compared to patients in the control group. Overall, significantly more adherent patients responded to treatment compared to non-adherent patients, regardless of method used to determine adherence. This large study demonstrates that treatment response increases when using an educational compliance programme and that a strong relationship between treatment adherence and response exists.