Telomerase is a ribonucleoprotein that synthesizes telometric DNA on chromosome ends, and may be related to the aging and immortality of cells. Recently, a telometric repeat amplification protocol (TRAP) assay for telomerase activity, using the polymerase chain reaction, was developed. We examined the limitations of TRAP assay by applying it to a cultured colon cancer cell line (COLO320) and 58 human cytological materials from body cavity effusions, and obtained the following results; i) The limits of the TRAP assay were 20-50 cells for the COLO320 cell line; ii) One COLO320 cell per 100 normal blood white cells was detectable; iii) Seventeen of 58 samples were positive for telomerase activity in this study. The sensitivity was 69% (9/13) and the specificity was 87.5% (28/32) between cytological diagnosis and telomerase activity; iv) Among 29 malignant cases, 15 were positive for telomerase activity, while there were 11 cytologically positive cases. The positive cases detected by the combination of cytology and telomerase activity accounted for 21 of the total 29 cases (72.4%). These results suggest that the measurement of telomerase activity in body cavity effusions may be useful as an adjunctive tool for cytological and clinicopathological diagnosis and that this technique is potentially applicable to remnant cytological materials.