An outbreak of leptospirosis occurred during the rainy season in the city of Mumbai, India. Out of 169 suspected cases, 74 (43.7%) were determined serologically positive by microagglutination test (MAT) carried out with a battery of eight pathogenic serovars, while 78 (46.1%) were shown positive for IgM antibodies to leptospira by enzyme-linked immunosorbent assay. On the basis of MAT, serovar Copenhageni accounted for 66 (89.1%) out of the 74 cases admitted during the period of the outbreak. Myalgia, conjunctival suffusion, cough with hemoptysis, icterus, and oliguria were significantly more common in patients whose samples were determined positive by MAT. The presence of pulmonary signs and symptoms and renal failure were significantly associated with mortality in patients presumed to be suffering from leptospirosis.