Objective:To investigate the prevalence of leptospirosis among patients from within and outside Kolkata, India, attending the Calcutta School of Tropical Medicine, for treatment during August 2002 to August2008.Methods: The leptospirosis cases were determined on the basis of clinical, epidemiological, and biochemical factors, and were tested for leptospiral antibodies using IgM ELISA. Serum samples with absorbance ratio ≥1.21were interpreted as reactive.Results: The commonest presentation involved fever, headache and jaundice. The male-female ratio was 61:46. A total of65(64.20%) cases had abnormal liver and renal functions respectively, and57.1% had both the abnormalities. The highest incidence(75, 35.04%) was recorded in September-October followed by July-August(53, 24.77%). The reactive cases had absorbance ratios between 1.21 and8.21, and53 showed equivocal result, while IgM non reactivity were seen in 90 patients (absorbance ratios 0.10-0.90). The patients responded to treatment with parenteral antibiotics, penicillin, ceftriaxone and cefotaxime; follow up did not reveal case fatality.Conclusions:The cardinal signs of leptospirosis help in making clinical diagnosis, but in any hyper-endemic situation any patient reporting with acute fever and signs of pulmonary, hepatic or renal involvement should be suspected to have leptospirosis and investigated accordingly. Increased awareness, and early diagnosis and treatment, can reduce mortality due to leptospirosis.