The wide variety of clinical syndromes produced in man by infection with members of the genus Leishmania has caused a great deal of confusion for many years, and has proved a serious obstacle to the rational classification of the leishmaniases.The situation has been complicated still further by the morphological identity of many species of Leishmania and by the behavioural similarities in vitro and in laboratory animals of species and strains producing distinct clinical or epidemiological patterns in humans. There has been a general failure to use standardized, comparable, and reproducible techniques in experimental studies of the various species of Leishmania; in particular, with one or two notable exceptions, there has been failure to adopt quantitative methods when studying Leishmania and the leishmaniases.This paper therefore attempts to classify the leishmaniases from clinical and epidemiological standpoints, and illustrates the provisional classification adopted with special reference to the situation in the USSR.