Background. A randomized controlled trial of problem-solving treatment, antidepressant medication and the combination of the two treatments found no difference in treatment efficacy for major depressive disorders in primary care. In addition to treatment outcome, the trial sought to determine possible mechanisms of action of the problem-solving intervention.

Method. Two potential mechanisms of action of problem-solving treatment were evaluated by comparison with drug treatment. First, did problem-solving treatment work by achieving problem resolution and secondly, did problem-solving treatment work by increasing the patients' sense of mastery and self-control?

Results. Problem-solving treatment did not achieve a greater resolution in the patients' perception of their problem severity by comparison with drug treatment, neither did problem-solving treatment result in a greater sense of mastery or self-control.

Conclusions. The results from this study did not support the hypotheses that for patients with major depression, by comparison with antidepressant medication: problem-solving treatment would result in better problem resolution; or that problem-solving treatment would increase the patients' sense of mastery and self-control.