Leptospirosis is a systemic disease affecting humans and animals, and pigs are generally considered the reservoir host species for the serovars Pomona, Bratislava and Tarrasovi. Endemic infections in swine herds generally remain subclinical, as do the vast majority of leptospire infections. However, when a susceptible breeding herd is infected for the first time or its immunity is compromised, considerable losses can occur due to abortion, stillbirths, weakly piglets or infertility. Infections in pigs caused by other serovars tend to occur only incidentally, vary regionally, and depend on other reservoir hosts, primarily rodents. Leptospires persist in porcine kidneys, and the Bratislava serovar, in the genital tract; it is excreted in urine and genital fluids. Leptospirosis is transmitted by direct or indirect contact with an infected animal. Fundamental research on porcine leptospirosis was conducted in the 1970s and 1980s. However, despite subsequent application of the most recent molecular biological methods, the pathogenesis of porcine leptospirosis is still largely unknown, and research results from the last 25 years on its incidence are very heterogeneous, due not only to regional differences but also to differences in the evaluation of diagnostic and population studies. Serological testing of pigs showed serovar prevalences ranging between as much as 16.3% (Pomona) and generally no more than 2.9% (Tarassovi), whereas antibodies against Bratislava were found in as many as 41.8% of pigs tested during the last 20 years, as in previous studies, indicating that this remains the most prevalent serovar.