Human telomerase reverse transcriptase (hTERT) expression level may not always correlate with telomerase activity. Although the positions of the spliced sites suggest that many of the variants do not code for functional reverse transcriptase, the functions of the spliced variants of hTERT are unknown. We analyzed hTERT splicing patterns with respect to telomerase activity in breast cancer. We examined telomerase activity by telomeric repeat amplification protocol (TRAP) assay and detected spliced variants of hTERT by reverse transcription-polymerase chain reaction (RT-PCR). Of 45 breast cancer patients, 38 (84%) were found to express telomerase activity and 41 (91%) expressed hTERT. In patients with telomerase activity, 14 (37%) expressed all four types of variants (full length, alpha, beta, and alpha/beta). Eleven patients (29%) expressed both the full-length and beta variant. Eight patients (22%) expressed the beta variant only and 3 (8%) expressed the full-length type only. When comparing telomerase activity to the expression of splicing variants, a tendency was found for lower telomerase activity in patients expressing the beta variant only (45 +/- 11) versus those expressing all four types (64 +/- 32) and those coexpressing the full-length type with the beta variant (61 +/- 22) (p = 0.06, respectively). In patients with both full-length and beta variants coexpression, increment of beta variant showed a decreased telomerase activity regardless of the full-length variant expression (p = 0.027). Telomerase activity changed with alternative splicing of the full-length and beta variants expression of hTERT in breast cancer.