The objective of our study was to demonstrate that additional antidepressant benefit occurs between weeks 4 and 6 in adult outpatients, even when dose is not increased. Response between weeks 4 and 6 was studied among depressed outpatients randomly assigned to imipramine, phenelzine, or placebo under double-blind conditions. Patients were selected for analysis only if they did not have a dose increase after the start of the fourth week of treatment (day 22). Eighty-eight patients met this condition. Conditional probability analysis was performed. Nonresponders to 4 weeks (28 days) of treatment had a significantly greater likelihood of responding by week 6 if they were on phenelzine rather than placebo. The same is probably true for patients on imipramine. In research and clinical care, 4 weeks is too short a trial of phenelzine to conclude a lack of efficacy. Four weeks is probably also too short a trial of imipramine.