Methylglyoxal (MG) is a reactive dicarbonyl intermediate of the glycolytic pathway. Increased oxidative stress is associated with conditions of increased MG, such as diabetes mellitus. Increased oxidative stress is due to an increase in highly reactive by-products of metabolic pathways, the so-called reactive oxygen species, such as superoxide anion, hydroxyl radical, hydrogen peroxide, nitric oxide and peroxynitrite. These reactive species react with a variety of proteins, enzymes, lipids, DNA and other molecules and disrupt their normal function. Oxidative stress causes many pathological changes that lead to vascular complications of diabetes mellitus, hypertension, neurodegenerative diseases and aging. In this review we summarize the correlation of elevated MG and various reactive oxygen species, and the enzymes that produce them or take part in their disposal, such as antioxidant enzymes and cofactors. The findings reported in various studies reviewed have started filling in gaps in our knowledge that will ultimately provide us with a clear picture of how the whole process that causes cellular dysfunction is initiated.