To investigate the telomerase activity in human lung cancer and its clinical significance. Sixty human lung cancer specimens (NSCLC 45 , SCLC 15) , seven tuberculosis specimens and seven inflammatory pesudotumor were examined for telomerase activity by means of TRAP (telomeric repeat amplification protocol) based on polymerase chain reaction. In NSCLC , telomerase positivity was 73. 3 %(33/ 45) with weak to moderate activity (172 TPG) , whereas 14 of 15 SCLC (93. 3 %) were positive with strong activity (288 TPG) , and all tuberculosis and inflammatory pseudotumor were telomerase negativity. The telomerase activity was significantly higher in SCLC than that in NSCLC ( P < 0. 05) , and was closely associated with the differentiation , size of the cancer , lymph node metastasis , and TNM staging ( P < 0. 05) . The results suggest that telomerase may be regarded as one of the prognostic factor in lung cancer.