Little is known about the association between antidepressant treatment-emergent adverse events and symptom nonremission in major depressive disorder. The objective of the current analysis was to determine whether particular baseline symptoms or treatment-emergent symptoms (adverse events) during the first 2 weeks are associated with nonremission after 8 weeks of treatment with a selective serotonin reuptake inhibitor (SSRI).Outpatients clinically diagnosed with nonpsychotic major depressive disorder were recruited from 6 primary and 9 psychiatric care sites. Participants (n = 206) were treated with an SSRI antidepressant (citalopram [20-40 mg/d], escitalopram [10-20 mg/d], fluoxetine [20-40 mg/d], paroxetine [20-40 mg/d], paroxetine CR [25-37.5 mg/d], or sertraline [50-150 mg/d]) for 8 weeks. Remission was defined as having a score of 5 or less on the 16-item Quick Inventory of Depressive Symptomatology-Clinician-Rated at week 8, or using last observation carried forward. Adverse events were identified using the 55-item Systematic Assessment for Treatment Emergent Events-Systematic Inquiry completed by participants at baseline and week 2.Findings indicated that the emergence of adverse events of weakness/fatigue, strange feeling, and trouble catching breath/hyperventilation at week 2 were independently associated with lack of remission even after controlling for the potential confounders of baseline depressive severity, anxious symptoms, antidepressant medication, chronic depression, race, burden of general medical comorbidity, and time in study. Hearing/seeing things appeared to have a protective effect. In conclusion, during SSRI treatment, the adverse events of weakness/fatigue, feeling strange, and trouble catching breath/hyperventilation are associated with nonremission, possibly due to lower adherence, early attrition, difficulty increasing the dose, and reduced efficacy. Copyright © 2011 by Lippincott Williams & Wilkins.