Impaired delayed type skin sensitivity is supposed to be due to abnormalities in T lymphocyte functions. Lately it has been assumed that T cells play an important part also in the development of late allergic reactions in the skin and in the lungs. Patients with sarcoidosis, lung carcinoma or with uremia were skin tested with anti IgE antibodies to challenge the hypothesis that the same sort of T cell deficiency as impaired the delayed skin sensitivity also attenuated the late allergic skin reactions. All patients obtained an early reaction and with a size similar to that of normal controls. In most of the patients late reactions were seen, but the frequency varied between 67% for patients with uremia and 100% for those with sarcoidosis. The cancer patients had significantly smaller late reactions than the normal controls while patients with uremia or sarcoidosis had a normal size of their reaction. No correlation could be found between delayed reactions to PPD and late reactions to anti IgE. It is concluded that the type of lymphocyte abnormality giving impaired delayed type reactivity in e.g. sarcoidosis patients does not affect the allergic early or late skin reactivity. Testing for late allergic reactivity may be a new tool for evaluating the immune status of, for example, cancer patients.