Objectives: The purpose of this study was to evaluate the diagnostic ability of FDG PET/CT to predict the hormone-secretion status of lipid-rich adrenal tumors. Methods: This study included 29 lipid-rich (CT number <10 HU) adrenal tumors 2 cm or larger in diameter in 28 patients who underwent FDG PET/CT. The diagnoses were based on endocrine examinations, including adrenal venous sampling and subsequent surgical resection, or on the endocrinological and morphological imaging follow-up during a period of at least 6 months. The FDG uptake of the adrenal tumors was evaluated semi-quantitatively using maximum standardized uptake values (SUVmax) and a ratio of the adrenal SUVmax compared to the liver SUVmax (SUVratio) was used for comparison. The statistical significance of differences was assessed using the Mann-Whitney U test, and a p value <0.05 was considered to be statistically significant. Results: The lipid-rich adrenal tumors were proved to be 16 non-hormone-secreting tumors (15 adenomas and one myelolipoma) and 13 hormone-secreting tumors (five subclinical cortisol-producing adenomas, six aldosterone-producing adenomas and two adenomas that produced both cortisol and aldosterone). None of the patients had pheochromocytoma or a malignant adrenal tumor. The SUVmax (median, range) of the hormone-secreting tumors (3.2, 2.0-8.3) was higher than that of the non-hormone-secreting tumors (2.4, 1.8-3.3) (p < 0.05). Similarly, the SUVratio of the hormone-secreting tumors (0.95, 0.70-3.10) was higher than that of the non-hormone-secreting tumors (0.72, 0.54-0.95) (p < 0.01). There was no significant difference in the tumor diameter between the two groups (p = 0.8). The sensitivity, specificity and accuracy of FDG PET/CT for differentiating hormone-secreting tumors from non-hormone-secreting tumors were 0.69, 0.81 and 0.76 for cutoff SUVratio of 0.8, and were 0.46, 1 and 0.76 for the cutoff SUVratio of 1.0, respectively. Conclusions: A lipid-rich adrenal tumor presenting increased FDG uptake compared with that of the liver is likely to be a hormone-secreting adenoma. Therefore, additional endocrinological investigations are strongly recommended when an FDG-avid lipid-rich incidentaloma is detected on FDG PET/CT.