We determined the prevalence of markers of atopy and asthma among 1, 199 rural secondary school students ages 12 to 19 yr. Subjects identified as having been raised on a farm and half as many subjects without regular exposure to a farming environment from the same school class completed a respiratory symptom questionnaire and underwent allergy skin tests and a methacholine bronchoprovocation test. Current wheeze, airways hyperresponsiveness (AHR), and skin test positivity to inhaled allergens were all significantly less common among adolescents raised on the farm and these differences were especially pronounced in girls. After adjusting for gender and current smoking, the odds ratios for being raised on a farm were: 0. 70 (95% CI 0.52 to 0.95) for current wheeze; 0.59 (95% CI 0.37 to 0. 95) for asthma, defined as the concomitant occurrence of wheeze and AHR; and 0.58 (95% CI 0.46 to 0.75) for atopy defined as a positive reaction to any one of 24 common inhaled allergens. These associations were also not significantly altered by adjusting for the difference in the number of siblings.