AIMS/BACKGROUND: Telomerase, an enzyme associated with cellular immortality, is expressed by most malignant tumours, but is inactive in normal somatic cells except for male germ cells and proliferating stem cells. Thus, the measurement of telomerase activity in tissue samples may provide useful diagnostic and prognostic information. The aim of this study was to determine whether telomerase expression is useful for the detection of occult malignant cells in lymph nodes. METHODS: Telomerase activity was compared with histological findings in 123 surgically removed lymph nodes submitted for routine or frozen section diagnosis. Telomerase activity was measured using a modified, semi-quantitative PCR-based telomeric repeat amplification protocol (TRAP). The assay was adapted for single 5 microns OCT embedded cryostat sections. In either fresh tissues or cryostat sections, normalised activity was linear when compared with protein concentration. Furthermore, using an in situ hybridisation method, the human telomerase RNA (hTR) component was measured in a subset of negative and positive nodes. RESULTS: Most (96%) of the 97 histologically negative nodes expressed low levels of activity (mean value of positive samples = 3.0 units/microgram protein) which may be derived from activated lymphocytes that express telomerase activity. All 26 malignant nodes (17 metastases, nine lymphomas) expressed telomerase (mean value = 17.8 units/microgram protein). The rank order levels between the two groups differed significantly (p = 0.0002). In situ results showed clearly that the hTR was expressed relatively highly in metastatic cancer cells and at lower levels in germinal centres of secondary follicles. CONCLUSIONS: Although expression of telomerase by activated lymphocytes may limit its usefulness, measurement of enzyme activity combined with detection of hTR using in situ hybridisation may assist in the histopathological diagnosis of lymph nodes.