Telomerase is a ribonucleoprotein polymerase that synthesizes telomeric DNA (TTAGGG) repeats. Previously, we have studied the effect on telomerase enzyme of several cis-platinum(II) complexes bearing aromatic amines as bulky carrier groups. All these complexes possess cis-geometry, according to the Cleare and Hoschele's rule. Since recent reports have dealt with the anti-cancer activity of trans-platinum compounds, in this study we have investigated the Farrell's prototypical trans-[Pt(Cl)2(pyridine)2], hereafter called trans-PtPy, in order to understand whether it may possess any anti-telomerase activity. The trans-PtPy has low water solubility and requires dimethyl sulfoxide (DMSO) as co-solvent, thus making the biological tests problematic. The effect of trans-PtPy on MCF-7 cell line concerning log-term telomerase inhibition, telomerase-related gene expression, viability, and apoptosis was evaluated. In a cell-free biochemical assay, trans-PtPy showed significant and dose-dependent inhibition of semi-purified telomerase. The bulk of data indicate that trans-PtPy acts as a non-properly selective anti-proliferative agent, although it shows an initial telomerase inhibitory effect. Telomere length reduction seems not to be the main mechanism causing the observed cell apoptosis. For comparison purpose, results on cis-[Pt(Cl)2(pyridine)2], hereafter cis-PtPy, are reported.