Leptospirosis is a bacterial infection found widely among animals, which can also infect humans. Human disease results in fever, with acute damage to the liver and kidney occurring in severe cases. Around 58,900 deaths are estimated to occur worldwide from leptospirosis. In Sri Lanka, where the disease is widely prevalent, leptospirosis is commonly seen as a disease of farmers. Humans are infected through contact with rat urine, especially during paddy cultivation. In recent times, increasing numbers of cases are being reported in urban and semi-urban areas, as overcrowding, flooding, and poor sanitation provide an environment conducive to the transmission of leptospirosis. There are many serological types of leptospirosis, and the prevalence of these different serological variations (serovars) differ according to locality and animal host. The disease manifestations vary depending on, among other factors, the infecting serovar. Leptospirosis is commonly diagnosed using the microscopic agglutination test (MAT), which tests for both IgM and IgG antibodies. There is little information available about the baseline presence of antibodies to leptospirosis in community settings. In this study, a representative sample from the community was studied to determine the seroprevalence of leptospirosis (presence of antibodies to leptospirosis), from the district of Colombo in Sri Lanka. The Colombo district represents an urban/semi-urban area of Sri Lanka, where overcrowding is common, and flooding takes place often. Leptospirosis has shown a sharp increase in the Colombo district in recent years. The aim of this study was to determine two important aspects; the patterns of seroprevalence of common serological types, relating them to geographical and other risk factors, and to determine baseline cut-off values for antibody levels in the community. The latter is of great importance when using MAT to diagnose patients presenting with acute disease. This study found a relationship between the presence of seroconversion to pathogenic Leptospira types and high risk occupational exposures, environmental exposures including exposure to floods, bathing in rivers and lakes, using well water for bathing, contact with stagnant water, propensity to skin injuries, presence of rats in the vicinity, and proximity to water sources. It was also found that seropositivity for pathogenic Leptospira was lower than that for non-pathogenic species, reinforcing the importance of performing serovar-specific MAT for diagnosis. The cut-off titres for different serovars were found to differ, and were lower than the titres of ≥1/400 or ≥1/320 which are currently recommended by the World Health Organization and by local reference laboratories, respectively. This finding has important implications when determining the MAT titre cut-off for confirming the diagnosis of acute leptospirosis in patients with a suggestive clinical presentation.