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Background: Adrenal incidentalomas are mostly benign nonfunctioning adrenal tumors (NFATs) or adenomas causing mild autonomous cortisol excess (MACE), but their natural history is unclear., Purpose: To summarize the follow-up data of adults with NFAT or MACE to determine the proportions of tumor growth, malignant transformation, and incident changes in hormone function; the prevalence of incident cardiometabolic comorbid conditions; and mortality., Data Sources: MEDLINE, Embase, Cochrane, and Scopus (January 1990 to February 2019) and bibliographies of identified articles, without language restriction., Study Selection: Studies that included 20 or more conservatively managed patients with NFAT or MACE and reported outcomes at baseline and after at least 12 months of follow-up., Data Extraction: Pairs of reviewers extracted outcomes and assessed methodological quality., Data Synthesis: Thirty-two studies reported outcomes of 4121 patients with NFAT or MACE, 61.5% of whom were women; the mean age was 60.2 years, and mean follow-up was 50.2 months. Mean tumor growth was 2 mm over 52.8 months. Clinically significant tumor enlargement (≥10 mm) occurred in 2.5% of patients, and none developed adrenal cancer. Clinically overt hormone excess was unlikely to develop (<0.1%) in patients with NFAT or MACE. Only 4.3% of patients with NFAT developed MACE, and preexisting MACE was unlikely to resolve (<0.1%). Hypertension, obesity, dyslipidemia, and type 2 diabetes were highly prevalent (60.0%, 42.0%, 33.7%, and 18.1% of patients, respectively) and were more likely to develop and worsen in MACE than NFAT. New cardiovascular events were more prevalent in MACE (15.5%) than NFAT (6.4%). Mortality was 11.2% and was similar between NFAT and MACE., Limitation: Evidence was scarce, and definitions of MACE and comorbid conditions were heterogeneous., Conclusion: During follow-up, NFAT and MACE do not show clinically relevant changes in size or hormonal function, but they may carry an increased risk for cardiometabolic comorbid conditions., Primary Funding Source: None.