Abstract. Leptospirosis is a zoonotic disease of worldwide distribution caused by spirochetes of the genus Leptospira. Humans are infected through direct contact with infected animals or through exposure to fresh water or soil contaminated by infected animal urine. Leptospirosis is characterized by acute fever that can be followed by a more severe, sometimes fatal illness that may include jaundice and renal failure (Weil's disease), meningitis, myocarditis, hemorrhagic pneumonitis, or hemodynamic collapse. To identify potential risk factors for leptospirosis in Thailand, we conducted a matched case-control study in Nakornratchasrima Province of the northeastern region. Fifty-nine cases and 118 controls were included in the study. Four activities in the two weeks prior to illness were independently associated with leptospirosis infection: walking through water (odds ratio [OR] ϭ 4.9, 95% confidence interval [CI] ϭ 1.7-14.1), applying fertilizer in wet fields for more than 6 hr a day (OR ϭ 3.4, 95% CI ϭ 1.5-7.8), plowing in wet fields for more than 6 hr a day (OR ϭ 3.5, 95% CI ϭ 1.1-11.6), and pulling out rice plant sprouts in wet fields for more than 6 hr a day (OR ϭ 3.1, 95% CI ϭ 1.02-9.3). Identification of these risk factors on admission might prove useful for early diagnosis and treatment of leptospirosis in Thailand.