Giardiasis is the most frequently reported diarrheal disease in northern New England. A case-control study of endemic giardiasis and environmental risk factors among residents of New Hampshire involved 273 cases from the state's 1984 disease registry and 375 controls. Giardiasis was associated with a shallow dug well as a residential water source (odds ratio [OR] = 2.4; 95% confidence interval [CI], 1.3-47.0), a recent history of drinking untreated surface water (OR = 3.4; CI, 2.1-5.5), a history of swimming in a lake or pond (OR = 4.6; CI, 2.4-86.0) or swimming in any natural body of fresh water (OR = 4.0; CI, 2.3-70.0), contact with a person thought to have giardiasis (OR = 2.3; CI, 1.4-36.0), and recent contact with a child in day care (OR = 1.5; CI, 1.0-2.1). Multivariate modeling supported these associations. Shallow wells, relatively common in New Hampshire, have not previously been established as important sources of giardiasis.