The clinicopathologic and histopathologic features of leukemic gingival and cutaneous "infiltrates" were studied in 1,076 adults hospitalized for cancer chemotherapy. Leukemic gingival hyperplasia was present in 3.6 percent and leukemia cutis in 3.1 percent of the patients. Only 7.6 percent of those with leukemic "infiltrates" had simultaneous gingival and skin involvement. Histologically, the lesions consisted of extravasated and proliferating monocytoid or myeloid cells. The infiltrative-proliferative tendency was most pronounced in patients with, in a sharply descending order, acute monocytic leukemia, acute myelomonocytic leukemia, and acute myelocytic leukemia. Formation of lesions in the skin and gingiva was unrelated to sex, age, and white blood cell count. Gingival lesions were particularly prone to infectious complications in patients with poor oral hygiene. Aside from the morphologic predisposition, the propensity for adults with leukemia to develop chemotherapeutically reversible malignant mucocutaneous lesions was highly individualistic and unpredictable.