Cryptosporidium infection is an important cause of diarrhea in humans and livestock; no effective therapy is known. A self-administered questionnaire and an ELISA were used to assess the risk of exposure to cryptosporidia among 70 dairy farmers and 50 who were not dairy farmers in Wisconsin. Dairy farmers (44.3%) were more likely to be seropositive for cryptosporidia than were other persons (24.0%; relative risk = 1.9). Among dairy farmers, age > or = 50 and use of a canister method of milking were associated with seropositive status. Among persons who were not dairy farmers, feeding or milking cows was associated with being seropositive. These findings suggest that dairy farmers and other persons who have contact with cattle are at greater risk of Cryptosporidium infection than are persons who do not have such contact. Identification and avoidance of farming practices associated with Cryptosporidium infection may reduce the risk of infection among dairy farmers.