Treatment strategies for treatment-resistant depression (TRD) involve either switching to another antidepressant, augmenting the first-line treatment with various pharmacologic agents, or switching to a different medication in combination with another agent. This study includes 2 phases: an open-label treatment phase and a double-blind maintenance phase, during which neither the patient nor the physician knows whether risperidone or placebo is administered. The open-label phase has an initial period of 6 weeks during which patients with TRD receive only citalopram, a selective serotonin reuptake inhibitor (SSRI). Risperidone is then added to the treatment regimen for 4 weeks to evaluate its short-term effect in augmenting the therapy. The double-blind phase last for 6 months and evaluates the relapse prevention of continued risperidone augmentation therapy compared with placebo augmentation of the SSRI. Targeted daily doses of citalopram and risperidone are adjusted for younger adults (18 to 54 years of age) and older adults (55 to 85 years of age). Assessments of effectiveness include the Montgomery-Asberg Depression Rating Scale (MADRS); Hamilton Rating Scale of Depression (HAM-D); Clinical Global Impression of Severity (CGI-S); response rate, determined by the proportion of patients showing >=50% improvement on MADRS total score; and relapse, evaluated by changes in CGI or HAM-D scores. Safety evaluations include the incidence of adverse events throughout the study, vital signs (pulse and blood pressure) and weight, and clinical laboratory tests (hematology, biochemistry, urinalysis) at specified intervals. The study hypothesis is that risperidone augmentation of SSRI therapy will show significant improvement in symptoms of depression compared with SSRI monotherapy and that risperidone augmentation will show better time-to-relapse than placebo augmentation in patients with TRD. Once daily, oral tablets of risperidone (0.25 milligram[mg], 0.5mg, 1.0mg, and 2.0mg), citalopram (20mg and 40mg) or placebo. Risperidone for 30 weeks; 0.25-2.0mg for younger adults, 0.25-1.0mg for older adults. Citalopram for 36 weeks; 20-60 mg for younger adults; 20-40 mg for older adults.