We measured the number of Langerhans cells in clinically normal epidermis and the number of T4 lymphocytes in the plasma in 24 patients infected with HIV, and related these to the stage of the disease. A correlation is demonstrated between the numbers of Langerhans cells and T4 lymphocytes. Numbers of Langerhans cells were lower in patients with disease stages III and IV than in those with disease stage II. Thus Langerhans cell numbers could be of prognostic value in this disease.