A prospective study of cancer incidence in a cohort examined for allergy.

In order to assess the association between atopy and cancer risk, a cohort of 6593 skin-prick-tested patients was established. Among atopic subjects, no overall increased cancer risk was found, but the incidence of both breast cancer (standardized incidence ratio (SIR) 2.50, 95% CI 1.01-5.16) and malignant lymphomas (SIR 4.40, 95% CI 1.20-11.3) was significantly enhanced. Atopic subjects with asthma showed a decreased overall cancer risk (SIR 0.73, 95% CI 0.27-1.60), as compared with the other asthmatic subjects (SIR 1.46, 95% CI 1.03-2.04). The cancer risk for subjects with rhinitis was near unity (SIR 1.11), irrespective of whether the subjects were atopic or not. An almost significant risk increase for cancer was observed among subjects with urticaria (SIR 1.70, 95% CI 0.99-2.80). Our results support neither the original hypothesis of an overall cancer protective effect of atopy, nor that of an opposite effect; rather, they strengthen the view that the association between atopic diseases and cancer is complex.