The aim of this review was to describe and determine the oral manifestation of DM and influences of periodontological treatment on DM. Diabetes mellitus (DM) is one of the most serious diseases of metabolism. Long-term consequences of hyperglycemia are very heterogeneous, and affect practically all tissues and organs of organism. Classical signs and symptoms of DM are polyphagia, polyuria, polydipsia, physical weakness, and decreased immunity against infections. Untreated and fully developed DM results in numerous complications, of which the most serious include nephropathies, retinopathies, myopathies, neuropathies, cardiovascular diseases, bad wound healing and disorders of microvascularity and macrovascularity. Oral manifestations of DM are of different types and they affect various tissues of this region. Summarizing and comparing the literature data were used to obtain these goals. From the etiopathogenetic viewpoint, we can state that the so far best-investigated oral complication is that of diabetic periodontitis and its consequences, including early teeth loss. Uncontrolled hyperglycemia deteriorates the periondontal status to the extent of developing into a clinical picture of diabetic periodontitis. On the other hand, it is to be noted that not all researchers have confirmed that the treatment of periodontitis brings about a statistically important improvement in diabetic markers, mainly HbA1c. It is necessary to continue in these studies (Ref. 34).