The telomeric repeat amplification protocol (TRAP) assay was used to measure telomerase activity in body cavity fluid from 10 ovarian cancer patients (ascites 9; pleural fluid, 1), ascites and peritoneal washings from eight uterine corpus cancer patients, and ascites from one with cancer of the uterine cervix. Telomerase activity was observed in five of six (83.3%) samples with positive cytology, one of four (25%) samples with suspicious cytology and one of nine (11.1%) samples with negative cytology. A high level of activity was observed in samples containing large numbers of blood-cell components, which could be removed without inactivating the telomerase by treating the samples with Nycodenz (N,N1-Bis (2,3-dihydroxypropyl)-5-[N-(2,3-dihydroxypropyl) acetamide] 2,4,6-triiodo-isophtalamide). In two patients with ovarian cancer treated with anticancer drugs, 5 and 7 days after treatment, intracellular vacuoles and multinucleation were observed in ascites tumour cells, and telomerase activity decreased. At 14 to 21 days after treatment, the ascites tumour cell morphology was the same as before treatment, and telomerase activity rose once again. The TRAP assay is a sensitive method of detecting telomerase in cytological material and may provide a useful adjunct to cytological diagnosis.