Immunocompetence and prognosis are related in solid tumors, malignant lymphomas, and acute leukemia. Among the parameters of immunocompetence vigorous delayed-type hypersensitivity responses to recall antigens or to primary immunization with Keyhole limpet hemocyanin, vigorous in vitro lymphocyte blastogenic responses to mitogens such as PHA, and relatively high B-lymphocyte levels, all correlate with a good prognosis. The spectrum of immune reactivity as measured by established delayed-type hypersensitivity to recall antigens and in vitro blastogenic responses to mitogens and antigens is similar in melanoma patients and their nontumor-bearing spouses. In melanoma, only patients with widespread inoperable metastatic disease show severe immunological deficiency and this is selective for certain antigens. There are highly significant differences in response to specific antigens when patients with melanoma and lung cancer are compared. Immunotherapy with BCG and C. parvum can boost immunocompetence as measured by recall DTH skin testing. However, the relationship between the initial immunocompetence and prognosis still holds in patients receiving BCG immunotherapy to prevent recurrence of melanoma. These data indicate that a broader survey of immunological reactivity in cancer patients is needed, that immunological testing is useful in cancer prognosis clinically, and that the results of immunological testing can be used to evaluate therapy and to indicate new pathways for improved treatment.