Trisomy 7 is a frequent aneuploid change in lymphomas, adenocarcinomas, and malignant mesenchymal and neurogenic tumors. Moreover, it has been observed in cultured and uncultured non-neoplastic cells from brain, kidney, liver, lung, and atherosclerotic plaques, among other tissues, opening debate on the role of this change in normal and neoplastic tissue. We used nonradioactive in situ hybridization (ISH) with a biotinylated chromosome 7-specific alpha-satellite DNA probe to seek an extra copy of chromosome 7 in ascitic and pleural fluid interphase cells from 26 donors. The donors comprised 24 patients with nonmalignant clinical history, one patient with non-Hodgkin's malignant lymphoma (positive control), and one patient with chronic myeloid leukemia (CML, negative control). The highest frequency of fluid cells with three hybridization signals in patients without neoplasia was 0.5%, in contrast to the frequency of 40.5% noted in the fluid cells of the patient with non-Hodgkin's malignant lymphoma. The results demonstrate that the frequency of trisomic cells in pleural as well as in ascitic fluid is very low, making possible use of the cells in ascitic or pleural fluids in identification of malignancy.