BACKGROUND: Postoperative delayed paradoxical depression (PDPD) is a psychiatric condition described in patients without a history of mood disorders who undergo major surgery without complications and become clinically depressed. PDPD has been recognized in major surgical interventions, including coronary artery bypass surgery. We sought to determine the incidence and potential factors associated with PDPD after surgical treatment of unruptured intracranial aneurysms. METHODS: The cohort of 105 patients was derived from a prospective observational data set of 3788 consecutive cases of intracranial aneurysms accrued from 1991 to 2015. Starting in 2010, patients with PDPD were identified, and psychiatric treatment and outcomes were documented. Incidence of PDPD and baseline characteristics were analyzed. Multivariate logistic regression was performed to analyze associations of variables with PDPD. Patients with preoperative depression or bipolar disorder were excluded. RESULTS: Of 105 patients, 10.5% (n = 11) were found to have newly diagnosed major depressive disorder after surgical treatment of intracranial aneurysms. By univariate and multivariate analysis, the only significant difference between the 2 groups was full return to daily activities (P = 0.017 and P = 0.029, odds ratio = 0.06, 95% confidence interval [0.00, 0.70]), which was a result and not a cause of PDPD. All 11 patients with PDPD recovered fully, 9 after psychotherapy and/or pharmacotherapy and 2 without intervention. CONCLUSIONS: PDPD after uncomplicated unruptured aneurysm surgery can be surprising to the neurosurgeon and the patient and should be promptly identified and addressed to achieve a full recovery. PDPD can be interpreted as a mild variant of post-traumatic stress disorder.