Abstract Background Limited data describe cognitive deficits in patients with Cushing syndrome (CS). The impact of mild autonomous cortisol secretion (MACS) on cognition is unknown. Objective To determine the impact of MACS on cognitive function. Methods Single-center cross-sectional study of adults with MACS and age and sex-matched volunteers. MACS was defined as cortisol >1.8 mcg/dL following the dexamethasone suppression test (DST) in patients with an adrenal incidentaloma and no features of CS. We measured cognitive function through the NIH toolbox cognition battery (7 standardized tests of attention, episodic and working memory, language, executive function, and processing speed). T-scores corrected for age, sex, education, and race were used for analysis. All patients completed the SF-36 questionnaire. The frailty index was calculated as previously reported (Singh S. JCEM 2020). Results Participants included 50 patients with MACS (median age 61, range 54-68) and 50 volunteers (median age 60 years, range 54-68), with 60% women in both groups. In patients with MACS, median post-DST morning cortisol was 3.6 (range, 2.4-6.2), and median corticotropin was 8.5 (range, 5.9-13) pg/mL (normal 6-60). Patients with MACS had a higher median frailty index (median of 0.26 vs. 0. 05 in volunteers, p<0. 001) and lower quality of life scores on the SF-36 survey (median of 47 vs. 91, p<0. 001). Compared to volunteers, patients with MACS performed worse in the domains of attention and executive function (effective allocation of one's limited capabilities in an abundance of stimuli) (median T-score of 50 vs. 46, p=0. 03), cognitive flexibility (tested as the capacity to plan and monitor goal-directed activities) (median T-score of 61 vs. 55, p=0. 01). The fluid composite score (includes executive function, episodic memory, working memory, and processing speed) was lower in patients with MACS (mean of 53.2 vs. 48.9, p=0. 03). The total composite score was lower in patients with MACS when compared to volunteers (median T score of 50 vs. 54, p=0. 06). The total composite cognitive scores in patients of MACS were associated with the Frailty Index (R2 = 0.13, p = 0. 025) and SF-36 scores (R2 = 0. 08, p = 0. 05), but not post-DST cortisol. Conclusion Compared to volunteers, patients with MACS demonstrate cognitive impairment, particularly in the executive domain, which closely mimics changes observed with aging. These deficits do not correlate with the degree of cortisol excess. Presentation: No date and time listed