Leptospirosis, which is caused by pathogenic spirochetes of the genus Leptospira, is a zoonotic disease of global importance and an emerging health problem. We studied patients suspected of having leptospirosis in Kamphaeng Phet Province, Thailand. Of 106 patients with suspected leptospirosis evaluated at the provincial hospital, 69 (65%) were confirmed positive (titer ≥ 1:800 or ≥ 4-fold increase in titer) by microscopic agglutination testing. Seventy-seven percent (53) of the cases occurred during the rainy season (June through November). Sera reacted predominantly with Bratislava, Autumnalis, and Icterohaemorrhagiae serovars. The screening Leptospira Dip-S-Ticks test had poor sensitivity (32%) but a specificity of 100% compared with a Leptospira IgM enzyme-linked immunosorbent assay. Leptospirosis was found to be a frequently confirmed cause of morbidity in Kamphaeng Phet Province in those suspected of having the illness.