Several epidemiological studies have evaluated potential associations between allergy and risk of malignancy. It remains clear that the relationship between allergy and cancer is complex. Three hypotheses have been proposed to account for observed relationships: these are chronic inflammation, immunosurveillance, prophylaxis, and we propose adding a fourth: inappropriate T-helper 2 (Th2) immune skewing. Each of these attempts to explain either the increased or decreased risk of different cancer types in 'allergic' patients reported in the literature. All four hypotheses are based on known mechanisms of allergic inflammation and/or IgE antibody functions, and uphold the view of an immunological basis for the relationship between allergy and malignancies. This review summarizes and draws conclusions from the epidemiological literature examining the relationships between specific types of cancer and allergic diseases. Particular emphasis is placed on the most recent contributions to the field, and on consideration of the allergic immune mechanisms that may influence positive or negative associations.