There is controversy as to the role of immune deficiency, if any, in Whipple's disease. This report summarizes published data in regard to immune function in this disease. Thirty-two publications during the past ten years offer varying amounts of immunological data in 61 patients--50 males, 6 females, and 5 patients in whom the sex was not reported. There is no evidence for humoral immune deficiency in these patients. Secretory immunoglobulins are within normal limits. Intestinal mucosal plasma cells are decreased before treatment, but are normal after treatment. There are no abnormal deposits of complement or immunoglobulins within the intestinal mucosa. There is no evidence for autoantibody production. These patients invariably have lymphocytopenia prior to treatment and have a decreased percentage of T cells both before and after treatment. There is decreased responsiveness of lymphocytes to the mitogens PHA and Con A, before and after treatment. The cutaneous response to antigens is clearly diminished before treatment, improves somewhat after treatment, but is still significantly less than that seen in normal controls. There may be an increased association with HLA B27, which suggests that an abnormality in the cellular immune system promotes susceptibility to the Whipple bacillus.