Leptospires, world-wide distributed spirochetes, affect a great variety of mammalian hosts; several serovars belonging to the L. interrogans s.l. species can cause clinical manifestations in humans, becoming infected through the contact of skin cuts and mucous membranes with water and soil polluted by infected animals' urine; rodents serve as the main reservoirs but the epidemiological importance of pets and cattle, as leptospire shedder is increasing. While the infection remains endemic in tropical regions, there is a new epidemiological trend in developed countries where, with the typical seasonal pattern, sporadic cases and/or outbreaks occur related more to recreational activities and poor sanitation than to occupational activities. The sudden onset presents a "flu-like" syndrome; the course is usually characterised by two clearly defined stages. All of the variable clinical manifestations, often independent of the responsible serovar, arise from the effects of a general vasculitis. The prognostic factors associated with severe forms (renal failure, jaundice, haemorrhagies) are not defined. Within the first days of illness, the leptospires can be isolated from blood and cerebrospinal fluid; serological diagnosis relies on microagglutination, IFA and ELISA; PCR early in the course, before the appearance of specific antibodies, allows etiological diagnosis. Prompt treatment has an enormous impact on outcome.