Purpose: To determine the diagnostic performance of apparent diffusion coefficient (ADC) histogram analysis in diffusionweighted (DW) magnetic resonance imaging (MRI) for differentiating adrenal adenoma from pheochromocytoma. Materials and Methods: We retrospectively evaluated 52 adrenal tumors (39 adenomas and 13 pheochromocytomas) in 47 patients (21 men, 26 women; mean age, 59.3 years; range, 16-86 years) who underwent DW 3.0T MRI. Histogram parameters of ADC (b-values of 0 and 200 [ADC 200 ], 0 and 400 [ADC 400 ], and 0 and 800 s/mm 2 [ADC 800 ])-mean, variance, coefficient of variation (CV), kurtosis, skewness, and entropy-were compared between adrenal adenomas and pheochromocytomas, using the Mann-Whitney U-test. Receiver operating characteristic (ROC) curves for the histogram parameters were generated to differentiate adrenal adenomas from pheochromocytomas. Sensitivity and specificity were calculated by using a threshold criterion that would maximize the average of sensitivity and specificity. Results: Variance and CV of ADC 800 were significantly higher in pheochromocytomas than in adrenal adenomas (P < 0.001 and P 5 0.001, respectively). With all b-value combinations, the entropy of ADC was significantly higher in pheochromocytomas than in adrenal adenomas (all P 0.001), and showed the highest area under the ROC curve among the ADC histogram parameters for diagnosing adrenal adenomas (ADC 200 , 0.82; ADC 400 , 0.87; and ADC 800 , 0.92), with sensitivity of 84.6% and specificity of 84.6% (cutoff, 2.82) with ADC 200 ; sensitivity of 89.7% and specificity of 84.6% (cutoff, 2.77) with ADC 400 ; and sensitivity of 94.9% and specificity of 92.3% (cutoff, 2.67) with ADC 800 . Conclusion: ADC histogram analysis of DW MRI can help differentiate adrenal adenoma from pheochromocytoma. Level of Evidence: 3