A ten-year-old, male cat presented with recent loss of body weight, depression, vomiting, anorexia, polydipsia, and polyuria. General physical findings included depression, weakness, severe dehydration and a strong acetone odor on the breath. A complete blood count and serum biochemical profiles were leukocytosis, hyperglycemia (286 mg/dl), hypokalemia (2.6 mEq/L), hyponatremia, and high serum fructosamine (600 $mu$mol/L). In blood gas analysis the cat had acidosis (pH 7.127, p$CO_2$26.7 mmHg). In urinalysis glycosuria and ketouria were appeared. On the basis of clinical signs, serum chemistry, blood gas analysis and urinalysis, diabetic ketoacidosis was diagnosed. Treatment included subcutaneous administration of protamine zinc insulin (0.75 U/head) and intravenous administration of 0.9% saline. Potassium phosphate and sodium bicarbonate was added to the fluids. Serum fructosamine for assessment of glycemic control was measured on occasion calls. On day 296, the patient improved clinically and did not experience any problems resulting from diabetic ketoacidosis.