Telomerase, a ribonucleoprotein complex that includes the telomerase RNA component, the telomerase-associated protein (TP1), the telomerase catalytic subunit (hTERT), and the heat shock protein 90 (HSP90), is closely related to the malignant potential of human tumors. In pheochromocytomas (PC) it is very difficult to predict malignant potential by conventional histology or immunohistochemical and molecular markers. To test whether the expression of telomerase subunits is reflected in the malignant transition of PCs, we determined their mRNA and/or protein expression in 28 benign and nine malignant PCs and compared the results with telomerase activity. RT-PCR analysis revealed that TP1 was ubiquitously expressed. The telomerase RNA component was found in all malignant (100%) and in 13 of 28 (46%) benign PCs. In contrast, hTERT was clearly associated with aggressive biological behavior. All of the malignant (100%), but only two of 28 benign (7%) PCs expressed hTERT. HSP90 was increased in malignant PCs, but was also expressed at a lower level in benign tumors. High telomerase activity was measurable in hTERT-positive tissues only. Our data indicate that hTERT, HSP90, and telomerase activity are up-regulated in malignant cells of the adrenal medulla. The common expression of hTERT and telomerase activity thus represents an additional prognostic marker that may identify more aggressive tumors.