BACKGROUND: Leptospirosis is a worldwide zoonosis causedby Leptospira interrogans. Leptospirosis results in decreasedmilk production, abortion, stillbirth, infertility and mortality,which causes financial loss in the cattle industry. OBJECTIVES:The aim of this research was to perform a serological andbacteriological study of leptospirosis in 6 industrial dairy herdsand 3 feedlots with previous records of leptospirosis in Tehransuburbs in 2011-2012. METHODS: For the purpose of this study,408 blood samples from dairy cattle and 154 blood samples fromfeedlots were collected using sterile 10ml venoject vacutainersfrom tail vein. Two months later, 118 urine samples werecollected from 20% of the two groups of serological negative andpositive animals. All serum samples were serologically tested bymicroscopic agglutination test (MAT), a standard method forserological diagnosis of leptospirosis. The serum samples weretested for antibodies against five live antigens of Leptospirainterrogans serovars: Pomona, Grippotyphosa, Hardjo, Icterohaemorrhagiaeand Canicola. Urine samples were used forbacteriological isolation of Leptospira spp. RESULTS: Serologicalresults showed that 228 (40.6%) of animals had a positivereaction against one or more serovars. The most prevalentLeptospira serovars was Pomona 118 (40.3%) and the leastprevalent was Canicola 4 (1.4%). The most prevalent titer was1:100, and the highest titer was 1:1600. Also the mostseropositive cases were observed in 3 to 4-year-old cows.Bacteriological results revealed that in 11 (9.3%) urine samplesLeptospira spp. were isolated, all taken from one feedlot farm.According to the history taken from each farm, the main riskfactors were the presence of rodents and low hygienic conditionsof the farms. CONCLUSIONS: The results of this study revealedthat cows could have a major role in maintaining Pomona,Grippotyphosa and Hardjo serovars; indeed, they are a potentialzoonotic risk to slaughter house workers, meat inspectors,milkers and farmers.