Objectives: To test the hypothesis that childhood acute lymphoblastic leukemia (ALL) is associated with allergic disorders. Methods: We compared the histories of selected allergic disorders (asthma, hay fever, food or drug allergies, eczema, and hives) of 1842 cases of ALL with those of 1986 individually matched controls. The histories of the allergic disorders among siblings of cases and controls were also compared. Results: The combined history of any one or more of the five allergic disorders evaluated was associated with a significant reduced risk of ALL (adjusted OR = 0.7, 95% CI 0.6-0.8), as were histories of four specific allergic disorders (asthma, hay fever, food or drug allergies, and eczema). The combined history of any one or more of the five allergic disorders among any of the siblings of the study subjects also revealed a significantly inverse association (adjusted OR = 0.9, 95% CI 0.8-1.0). Conclusion: The results from this study, in agreement with most previous studies on adult cancer, suggest that allergic disorders may be associated with a reduced risk of childhood ALL.