Malignant transformation and tumor progression is followed by changing serum glycoprotein pattern. More important than glycoproteins are their terminal and preterminal carbohydratrests. N-acetylneuraminic acid (NANA) localized in terminal position is of special interest. Considerable variations in the serum of patients with malignancies gave rise to the study of NANA in 66 women with gynecologic carcinomas. In 30 healthy female controls there was a concentration of 2.0 +/- 0.3 mmol . l-1 In the sera of patients with cancers NANA-concentration before treatment was between 2.6 +/- 0.3 and 4.3 +/- 0.2 mmol . l-1. After therapy it decreases, sometimes it was persistent. In cases without recurrencies over a period of some years NANA-serum-concentrations were between 2.3 +/- 0.4 mmol . l-1 and 2.5 +/- 0.4 mmol . l-1, but in cases with recurrent or progressive tumor they increased to values between 4.3 and 5.0 mmol . l-1. The possible importance of NANA as a tumor marker in gynecologic oncology is discussed.