OBJECTIVE: To investigate the relationship between allergy and risk of breast cancer in women 45 years of age and younger. METHODS: Data were analyzed from a population-based case-control study of breast cancer in western Washington. Cases were women born after 1944 who were diagnosed with invasive breast cancer (n = 747) between January 1983 and April 1990. Controls (n = 958) were similarly aged women ascertained through random-digit dialing. Cases and controls were interviewed about their history of doctor diagnosed allergies, including detailed information on the specific types of allergies and the age of onset. Using logistic regression we examined the associations between allergy history and breast cancer. RESULTS: A history of allergies was associated with a reduced risk of breast cancer for women older than 35 (odds ratio (OR) = 0.77; 95% confidence interval (CI) = 0.60-0.99), but not for women 35 years or younger (OR = 1.30; 95% CI = 0.94-1.81). There was little difference in effect when age of first allergy onset was examined. No specific type of allergy was associated with breast cancer risk. CONCLUSION: Our results provide some evidence that a history of allergy may be associated with a reduced risk of breast cancer for women who develop breast cancer between 35 and 45 years of age. Future studies are needed to verify the relationship between immune responses and breast cancer risk.