Metformin is a biguanide. Due to its effects in suppressing the hepatic production of endogenous glucose and in increasing insulin sensitivity in adipose tissue and skeletal muscle, the agent is used particularly in type 2 diabetes mellitus and metabolic syndrome, in which insulin resistance is especially pronounced. Lactic acidosis is one of the most important side effects of metformin. A male patient, born in 1923, was admitted to the emergency unit of our hospital for sudden vertigo, weakness, dyspnea, cyanosis, and lethargy. His history data showed that the patient had been suffering from type 2 diabetes mellitus for 10 years and taking Glargin (insulin), 12 U/kg, once daily and Glucophage (metformin), 850 mg thrice daily. The patient's general condition was fair; stupor, time and spatial orientation were absent. Analysis of arterial blood gases showed the presence of metabolic acidosis, hypokalemia, hypoxemia, and hypercapnia. Thereafter the patient was transferred to the intensive care unit of the hospital; intubated and connected to a T-bird ventilation apparatus. On the following day, an analysis of arterial blood gases indicated the proximity of the results to their physiological parameters. Ventilation was stopped; and monitoring of the patient continued by following the T-shape type of ventilation discontinuation. There were no X-ray signs of pneumonia or pulmonary edema. On the same day, the patient was extubated and oxygen inhalation in a dose of L/min was continued through a mask. On day 4 since therapy was initiated, the patient's vital signs, serum sugar and lactate levels became normal. By determining a new treatment regimen, the patient was discharged from the intensive care unit. Dyspnea, acidosis, and hypoxia developed in the patient resulted from lactic acidosis caused by the use of metformin. It should be remembered that dyspnea, acidosis, and hypoxia, which suddenly developed in metformin-treated patients with type 2 diabetes mellitus, may be caused by lactic acidosis.