The total blood lymphocyte counts, T-cell counts, B-cell counts, percentage of T cells bearing Fc receptors for IgG (TG), lymphocyte responses to phytohaemagglutinin and concanavalin A and serum IgA levels were measured in 15 patients with hydatidiform mole, 32 patients with malignant trophoblastic disease, 30 pregnant controls, and 34 normal non-pregnant controls. The mean percentage of TG was similar in all four groups but the mean absolute number of TG in patients with malignant trophoblastic disease was significantly lower than that in normal non-pregnant controls, due to the lower mean lymphocyte count and T-cell count in the former group. An immune profile score based on the T-cell count, B-cell count, lymphocyte response to mitogens, and serum IgA level was useful in predicting the subsequent development of malignant trophoblastic disease in patients with hydatidiform mole but was not helpful in predicting resistance to chemotherapy in malignant trophoblastic disease.