Neopterin, a pyrazinopyrimidine compound, is a marker of activation of cell-mediated immunity. Urinary neopterin concentrations were measured in a total of 44 patients with uterine sarcomas. Pretherapeutic neopterin excretions were elevated in 78% (14/18). The mean neopterin concentration of patients with sarcomas was significantly higher (P less than 0.0001) than that obtained in women with benign myomas and healthy controls. Furthermore, a significant correlation of normal or raised neopterin concentrations with the clinical course of disease was found. These data suggest that urinary neopterin measurement might be a useful immunologic marker for women with uterine sarcomas.