The recent epidemiological trends of human leptospirosis in Italy were investigated using data collected for the years 1981-1985. A total of 626 hospitalized patients with clinical diagnoses of suspected leptospirosis were reported by hospital centers from several Italian regions. Epidemiological, clinical and seroimmunological data were collected in 517 of these cases and examined by the National Center for Leptospirosis. Serological findings in 33.5% of these subjects met the criteria for confirmation of the disease. In 21.8% of the subjects, low titer antibodies were detected, which possibly reflected previous leptospiral infections. An early antibiotic treatment of the current infection may also have lowered the seroimmunological response in some of these patients. In 59.3% of the confirmed cases, modes of transmission were allotted equally between accidental events and recreational or occupational activities. Drinking water from an open air fountain emerged as an uncommon mode of transmission; it was responsible for an outbreak of 33 cases of leptospirosis. In another 37.07% of the subjects, it was impossible to establish the mode of transmission. Respiratory or influenza-like symptoms were the only clinical signs of illness in 21.2% of the patients with confirmed leptospirosis. In comparison to the sixties and seventies, the prevalence of infecting serovars showed increasing incidence of infections due to serovars of the Javanica (11.0%) and Australis (11.0%) serogroups and an important decrease in the Bataviae serogroup infections (from 58.8% in rice-field workers in the forties to 0.6% in the years 1981-1985). Sejroe serogroup infections accounted for 4.5 per cent of confirmed cases of leptospirosis.(ABSTRACT TRUNCATED AT 250 WORDS)