Diabetes mellitus is among the diseases with great impact on health and society, not only for its high prevalence but also for its chronic complications and high mortality. The most precise method to investigate the prevalence of diabetes is by oral glucose tolerance testing. In Spain, the prevalence of diabetes in the 30-65 year-old population is estimated to be 6.5% among 30-to-65- year old, and 10.3% among the 30-to-89 year-old population. The ratio of known to unknown diabetes ranges from 1:3 to 2:3. The incidence of diabetes mellitus type 2 in Spain is 8/1000 persons per year, and the incidence of type 1 is 11 to 12 cases per 100,000 persons per year. The prevalence of chronic complications varies according to type of diabetes, time since onset and degree of metabolic control: neuropathy 25%, retinopathy 32% and nephropathy 23%. Diabetes is one of the most important causes of death in Spain, occupying third place for women and seventh for men.