We herein present the findings of a 42-year-old woman with either adrenal pheochromocytoma or intraadrenal paraganglioma that simultaneously secreted somatostatin, thus mimicking insulin-dependent diabetes mellitus. Pheochromocytoma was clinically diagnosed based on scintigraphy, elevated catecholamine levels, and finally a histopathological analysis of resected specimens. The patient had diabetic ketosis, requiring 40 U insulin for treatment. Following laparoscopic adrenalectomy, insulin therapy was discontinued and the urinary c-peptide levels changed from 5.5-9.0 to 81.3-87.0 μg/day. Histologically, somatostatin immunoreactivity was detected and the somatostatin levels were elevated in the serum-like fluid obtained from the tumor. Clinicians should be aware of the possible occurrence of simultaneous ectopic hormone secretion in patients with pheochromocytoma.