A 73-year-old Japanese woman was referred for examination of right flank pain and progressive hypertension. Abdominal CT incidentally detected a right adrenal mass 8 cm in size. The tumor exhibited isodensity by CT and contained high-intense lesion by T2-weighted MRI. Scintigraphy with (131) I-metaiodobenzylguanidine and (131) I-adosterol showed no abnormal uptake by whole body scan. Positron emission tomography scan with (18) F-2-fluoro-D-deoxyglucose demonstrated an exclusive uptake in the right adrenal mass. Adrenocortical hormone levels and catecholamine secretion were within normal range; however, the level of serum neuron-specific enolase (NSE) was found to be markedly high. After controlling systemic blood pressure with an alpha1-blocker, the right adrenal tumor was surgically removed, along with the right kidney and inferior vena cava which adhered to it. The tumor was pathologically proven to be leiomyosarcoma, which was immunohistochemically positive with alpha-smooth muscle actin and negative with CD57, S-100 and c-kit proteins. Notably, NSE protein was massively expressed in the resected tumor. After surgery blood pressure was controlled with regular medication and serum NSE levels have since normalized. The possibility of leiomyosarcoma should be kept in mind in adrenal incidentalomas with rapid growth and atypical radiological images. Our findings suggest that circulating NSE levels may be clinically useful for early detection of recurrence.