Over 45 years, anthropurgic foci of Leptospira infection have been differently active in the Republic of Mordovia; in its population, the incidence of leptospiroses being greater than the average federal rates. The peak incidence of leptospirosis was observed at 10-11-year intervals: 1962-1963, 1971-1972, 1983, 1994-1995, 2001-2002 when outbreak and group morbidity were registered. There were 2.6- and 2.4-fold seasonal rises in morbidity in June to September with its peak in July-August, respectively. Cases of infection were notified in all the districts and towns of the republic, and in all landscape-environmental areas; group and outbreak incidence was found in 6 districts and environs of the town of Saransk in 1997-2002. Males aged 20-49 years are a group of risk for Leptospira infection; its incidence rates in age groups 0-14 and above 60 years are less than the average republican ones. Children (aged 0-14 years) and adolescents (aged 15-19 years) actively participate and determine outbreak and group mortality rates. Infection occurs most frequently while bathing in open water reservoirs, while using water from other water springs, going to the forest to pick mushrooms, berries, etc., while being engaged in laying in hay, fodder, and other feeds for animals. During registration of infection in Mordovia, the etiological pattern of human leptospirosis constantly changed, in 1992-2002 there were prevalent diseases caused by Grippotyphosa (60.96%), Canicola (7.12%), Seiroe (6.73%), Australis (6.54%), and Cynopteri (5.96%), in 1997-2002, the proportion of Australis (11.2%) increased and that of Canicola (4.7%) decreased. The leptospira serogroups Icterohaemorrhagiae, Tarassovi, Seiroe were prevalent animals. There was no correlation of the etiological environment of leptospiras isolated from agricultural animals and human beings in 1997-2002, which proves the leading role of natural leptospirosis foci and rodents as a source of infection.