Laboratory animal workers are at high risk of developing occupational allergy. Little is known about the relationship between levels of exposure and the risk of developing laboratory animal allergy. A cross-sectional study was performed in 540 workers at eight facilities to quantify the exposure-response relationship for allergy to rats, while controlling for determinants like atopy, gender, and smoking. All participants completed a questionnaire, underwent skin prick testing with common and occupational allergens, and total IgE as well as occupational allergen-specific IgE antibodies were serologically measured. Personal air dust samples were taken during full-shift periods to estimate the rat urinary aeroallergen exposure levels. In the whole study population no clear exposure-response relationship was observed. However, in the group of workers with less than 4 yr of working experience with laboratory animals the prevalence rate of sensitization to rat allergens was clearly associated with exposure levels. The exposure-response relationship was steepest for workers with atopy-associated risk factors, i.e., self-reported allergy or sensitization to cats or dogs, or elevated total serum IgE. The prevalence rates of sensitization to rat allergens for these workers were about 15, 9.5, and 7.3 times higher in the high, medium, and low exposure group, respectively, compared with internal reference group.