Improvements in imaging have resulted in an increase in incidentally discovered adrenal tumors. The adrenal incidentalomas have been identified in at least 2%-3% of patients receiving abdominal computed tomography (CT), and pheochromocytomas are reported to occur in about 5.1%-23.0% of those patients. Only a few case reports of clinically silent pheochromocytomas have been published in the English literature. Herein we present 4 cases of pheochromocytomas as unforeseen adrenal masses on abdominal images. The characteristics of these tumors were that they (1) were noted to have no particular symptoms, (2) were confirmed biochemically and pathologically with immunophenotypic staining, (3) were benign in nature, and (4) produced apparent blood-pressure fluctuations during the operation. Three of these patients underwent a hypertensive crisis during surgery, and antihypertensives were applied transiently for blood pressure control. Two cases suffered from dramatic hypotension after the tumor had been excised, and intravenous fluid expanders and ephedrine were necessary to maintain adequate blood pressure. The above changes in blood pressure had no correlations with the size of the tumors. Although these tumors were clinically silent, they indeed were biologically active. Surgical resection of these tumors was indicated, and the risk of complications, such as hypertensive or hypotensive crises, may be no less than those of other typical tumors. Thus, such tumors should be managed with great caution.