Gastric cancer was discovered in a 72-year-old man during a regular medical examination. Preoperative laboratory testing showed the patient to have diabetes mellitus, with PG 228 mg/dl and HbA1c 10.1 %. The combination of acromegalic facial features, no inhibition of GH in a 75 g OGTT, and MRI findings of a space-occupying lesion in the pituitary gland yielded a diagnosis of acromegaly. The treatment of gastric cancer was given priority, and the treatment of acromegaly was started with a long-acting formulation of octreotide acetate 20 mg/month, which successfully controlled IGF-I until they reached levels within the normal range. Intensive insulin therapy and drug treatment were started to treat the patient's diabetes. Despite a decrease in the endogenous insulin secretion after starting octreotide acetate, the blood glucose levels were subsequently well-controlled. A positive result was found for the IGF-1 receptors of the patient's gastric cancer cells, thus suggesting a relationship with acromegaly. Six months after starting the administration of long-acting octreotide acetate, the patient underwent transnasal pituitary tumor resection, which alleviated the acromegaly, and the drug treatment for diabetes was stopped. In this communication we describe the interesting clinical course of this patient with gastric cancer, diabetes mellitus and acromegaly.