The purpose of this trial is to evaluate the efficacy and safety of risperidone versus placebo in subjects with Major Depressive Disorder with sub-optimal response to antidepressant therapy.

Many patients who suffer from Major Depressive Disorder (MDD) do not benefit or show only partial benefit from current psychotropic therapy. This clinical trial seeks to study the efficacy and safety of adjunctive treatment with risperidone in patients with MDD who failed to adequately respond to standard antidepressant treatment prior to this trial and who prospectively do not respond to adequate treatment with standard antidepressant therapy (SAD).

Patients entering the open label phase of the study will have already taken a commercially available SAD for a minimum of 4 weeks, and provided the dose was optimal, will continue on this dose of SAD throughout the four-week open-label and six-week double-blind phases of the trial.

If the dose in the four weeks prior to entering the study was not optimal (as per standard clinical practice), the dose will be increased to be within the optimal range and continued at that dose through the entire open-label SAD and double-blind phases.

During the double-blind phase, subjects receive an adjunctive dose of risperidone or placebo, once daily, added to their stable dose of SAD. The starting dose of risperidone or placebo is 0.25 mg for the first three days of the double-blind phase, and is increased to 0.5 mg for days 4 through 14. On the 15th day of this phase, the dose is increased to the target dose of 1.0 mg/day. If response to this dose is not optimal by day 29, it is increased to 2.0 mg/day and is maintained for the duration of the 6-week double-blind phase. This dose may be reduced once to 1.0 mg/day, but then must be maintained at that dose for the remainder of the study.

Inclusion Criteria:

• Understand and sign the informed consent form
• Age 18-65
• Healthy on the basis of Physical Exam
• Treatment with a single antidepressant 4 weeks prior to study start and willingness to maintain on stable dose of the same antidepressant throughout the study
• Current diagnosis of Major Depressive Disorder
• Judgement of the clinician that the subject has shown a sub-optimal response to the antidepressant

Exclusion Criteria:

• Presence of other serious medical illness(es)