From the Immunodeficiency Cancer Registry, it has appeared that there is an increasing frequency of neoplasia in individuals who have a selective absence of serum IgA. Approaching this question from another point of view, we have found that of 4,120 sera drawn in this cancer-oriented hospital, 12 sera had a total absence of IgA and 3 additional sera had less than 10 mg/dl. The incidence of IgA deficiency in a cancer hospital is thus 1 : 342 or 1 : 273, which is statistically similar to that previously found for other patient groups studied in the USA (average of two studies, 1 : 418), but it is substatistically increased over the incidence of IgA deficiency found in normal blood donors (average of five studies 1 : 1,677). Analysis of these sera by diagnostic categories showed that of 1,517 sera of patients with lymphoproliferative disorders, 6 were IgA deficient (frequency 1 : 253), and of 249 sera of patients with gastrointestinal neoplasm, 2 were IgA deficient (frequency 1 : 125). We conclude that, in the absence of IgA, certain organ systems, the gastrointestinal and lymphoid tissue may be at increased risk for malignant change and that the protective, anti-neoplastic role of IgA requires investigation.