Aim. Diabetes mellitus (DM) is known to be a risk factor in adverse outcomes and complications in many infectious diseases. In the combination of hemorrhagic fever with renal syndrome (HFRS) and DM there are mutually exclusive pathogenetic States - hyperosmolarity, characteristic of DM is layered on the reduction of osmotic blood pressure in HFRS. Under these conditions, the effect of one disease (DM) on the clinical manifestations of another (HFRS) is not obvious. The aim of the work is to find out the clinical features during hfps in patients with DM. Materials and methods. The study is based on the results of the retrospective studies "case - control" - studied the information contained in medical records of patient (form 003/u), who suffered HFRS in 2006-2018. The Selection of cards produced randomly. In the end, there were formed two groups: the first - 981 patient who suffered HFRS and had no signs of diabetes; the second, 33 patients who suffered HFRS on the background of previously existing (28 people), or the first identified (5) diabetes. Results. With a combination of HFRS and DM, a mild course of the disease is 2.5 times more common, there are no severe forms. Among this group of patients, complications are almost 10 times less common, less likely to develop infectious - toxic shock, acute kidney damage (class F by RIFLE), pulmonary edema. The combination of DM and HFRS is manifested by less high and prolonged fever, less high levels of urea and creatinine in the blood. Conclusion. HFRS in combination with DM is easier, apparently, high osmolarity of the blood is a stabilizing factor during the disease.