We report three cases of Cushing's syndrome (CS) with bilateral adrenal tumors. When bilateral adrenal tumors are encountered, a differential diagnosis is difficult to make, especially in the case of functioning bilateral adrenocortical adenoma. Adrenal scintigraphy has become a standard technique to determine the laterality of excessive hormone secretion; however, this examination results in bilateral adrenal activity in the functioning bilateral adrenocortical adenoma. Our three patients were diagnosed with adrenocorticotropic hormone (ACTH)-independent CS based on biochemical testing, and an abdominal computed tomography (CT) scan detected bilateral adrenal tumors. Adrenal scintigraphy showed bilateral adrenal activity in all cases. However, adrenal venous sampling (AVS) demonstrated three different hormone-excess patterns (case 1: bilateral cortisol-excess secretions; case 2: unilateral cortisol-excess secretion and bilateral aldosterone-excess secretions; and case 3: bilateral cortisol-excess secretions and bilateral aldosterone-excess secretions). Based on these findings, we could select optimal treatment for each case. Therefore, AVS is useful to obtain a definitive diagnosis and adequate therapy for CS with bilateral adrenal tumors.