BACKGROUND: Day-to-day functioning is impaired in major depressive disorder. Yet there are no guidelines to systematically assess these functional changes. This report evaluates prognostic utility of changes in activity impairment to inform clinical decision-making at an individual level. METHODS: Mixed model analyses tested changes in activity impairment (sixth item of Work and Activity Impairment scale, rated 0-10) at mid-point (week 6) and end of step 1 (weeks 12-14) in the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) trial (n = 2697) after controlling for depression severity [Quick Inventory of Depressive Symptomatology Self-Report (QIDS-SR)]. Interactive calculators for end of step 1 remission (QIDS-SR ≤5) and no meaningful benefit (<30% QIDS-SR reduction from baseline) were developed for participants with complete data (n = 1476) and independently replicated in the Combining Medications to Enhance Depression Outcomes trial (n = 399). RESULTS: Activity impairment improved independently with acute-phase treatment in STAR*D (F = 7.27; df = 2,2625; P < .001). Baseline to mid-point activity impairment change significantly predicted remission (P < .001, model area under the curve = 0.823) and no meaningful benefit (P < .001, area under the curve = 0.821) in the STAR*D trial. Adding activity impairment variables to depression severity measures correctly reclassified 28.4% and 15.8% remitters and nonremitters (net reclassification improvement analysis, P < .001), and 11.4% and 16.8% of those with no meaningful benefit and meaningful benefit (net reclassification improvement analysis, P < .001). The STAR*D trial model estimates accurately predicted remission (area under the curve = 0.80) and no meaningful benefit (area under the curve = 0.82) in the Combining Medications to Enhance Depression Outcomes trial and was used to develop an interactive calculator. CONCLUSION: A single-item self-report measure of activity impairment changes independently with antidepressant treatment. Baseline to week 6 changes in activity impairment and depression severity can be combined to predict acute-phase remission and no meaningful benefit at an individual level.