The detection of malignant cells in fine-needle aspirates (FNA's) using marker genes is hampered by the fact that these markers are only expressed by certain malignancies or lack sensitivity and/or specificity. Here we report the results of a prospective pilot study examining the expression of KOC (KH-domain containing protein over expressed in cancer), a novel onco-foetal gene, in 76 patients who underwent fine-needle aspiration for further diagnosis of abdominal lesions, aszites, cysts or cerebrospinal fluid. Aspirates were examined by cytology and by a KOC RT-PCR assay. KOC expression was a highly sensitive and specific indicator of malignancy. The KOC assay could be useful to facilitate screening for malignant disease and to improve the diagnostic accuracy of FNAs.