A history of respiratory atopy (seasonal hay fever or asthma) was sought among 218 patients with malignancy of endodermal origin (lung, gut, bladder, prostate), 104 patients with mesodermal malignancy (hematological, sarcomas, genitourinary system), 70 patients with ectodermal malignancy (skin and breast) and 303 age- and sex-matched controls. There was a significantly lower frequency of respiratory allergy in patients with endodermal malignancy when compared with their matched controls (6.4% compared to 13.2%, P < 0.005). There were no significant differences among the other groups. Patients with respiratory atopy appeared to have some degree of protection against developing malignancies of endodermal origin. This may relate to immunologic factors, an inherent difference in the endodermal cell layer in atopic individuals or as yet undetermined factors.