We report a rare case of Cushing's syndrome caused by bilateral cortisol-secreting adenomas in a 63-year-old man. Our preoperative diagnosis was based on endocrinological results and imaging findings. Laparoscopic adrenalectomy has become a standard technique for adrenal tumors; however, bilateral adrenalectomy results in postoperative adrenal insufficiency, necessitating lifelong steroid replacement. To preserve adrenal function, the left adrenal gland was completely resected, whereas the right adrenal gland was partially resected laparoscopically. Hydrocortisone supplementation was initiated at a dose of 30 mg/day and was slowly tapered. However, symptoms of adrenal insufficiency developed, and adrenal steroid secretion did not respond to exogenous adrenocorticotropic hormone. Bilateral cortisol-secreting tumors rarely cause Cushing's syndrome. The present study comprised few patients, and the utilized surgical procedures (i.e., total/partial adrenalectomy or bilateral total adrenalectomy) were not uniform. Few cases of bilateral adrenal-preserving surgery have been reported. However, our patient developed adrenal insufficiency after the oral cortisone supplementation was tapered. This report demonstrates that partial adrenalectomy does not necessarily preserve normal adrenocortical function. Therefore, careful postoperative observation is necessary for patients undergoing a partial adrenalectomy.