Introduction: Sensitisation, and the resulting allergy, to laboratory animals represents a special problem at work. Rat urinary proteins play a significant role as allergens. The aim of this study was to describe the risk factors for sensitisation to rodents and the development of bronchial hyperreactivity, and to formulate recommendations for occupational health and safety when handling laboratory animals. Method: A group of 35 volunteers who handled laboratory rats were compared with 28 controls whose work did not involve exposure to animals. In addition to a detailed anamnesis, chemical analysis of the specific IgE to rat urinary proteins and epithelia was carried out, together with a prick test for various environmental allergens. Non-specific bronchial hyperreactivity was determined by means of a methacholine provocation test. Results: In this collective, the people who were exposed to rats had a higher percentage of specific IgE antibodies to rat urinary proteins than to rat epithelia (9.5 % vs. 1.6 %). Sensitisation was more frequent in smokers than in non-smokers (p = 0.0087). In the collective the risk of bronchial hyperreactivity was markedly higher in the group of smokers (p = 00.87). A history of pollinosis (p = 0.0006) and a tendency for atopy (p = 0.0031) also had a positive effect on the incidence of bronchial hyperreactivity. No significant relationship was found between the occurrence of bronchial hyperreactivity and exposure to rats (p = 0.196). A comparison of the percentage frequencies indicated that there is a trend for exposure to rat urinary proteins to lead to bronchial hyperreactivity more frequently where there is nicotine consumption (27.6 % vs. 8.3 %), allergic diathesis (57.1 % vs. 11.1 %) and pollinosis (45.5 % vs. 11.1 %) than in non-smokers and volunteers without any tendency to develop allergies. The intensity of exposure to rat urinary proteins did not have any effect on the frequency of the development of bronchial hyperreactivity (odds ratio 1.313, 95 % CI: 0.191; 9.021). Conclusions: In cases of suspected allergy to laboratory animals the allergological differential diagnosis should include rat urinary proteins. According to our observations, significant risk factors for the development of bronchial hyperreactivity are nicotine consumption, a history of pollinosis and a tendency for atopy.