Telomerase is activated in most tumors, but suppressed in normal human somatic cells. Current evidence indicates that telomerase reactivation is a critical step in carcinogenesis, with a close relationship to apoptosis. The goal of this study was to investigate the levels and relationship of telomerase activity to apoptosis and its impact on the survival of Egyptian adult acute lymphoblastic leukemia patients. Telomerase activity was quantified by polymerase chain reaction (PCR) and detected by enzyme-linked immunosorbent assay (ELISA), while apoptosis was measured at the single-cell level by fluorescence in situ detection using flow cytometry in 15 control subjects and 40 acute lymphoblastic leukemia (ALL) patients at presentation. Telomerase activity in ALL patients was negatively correlated to apoptosis [percent and mean fluorescence intensity (MFI)] (p < 0.001 for percent and p < 0.001 for MFI) and to the 4-year survival rate (p < 0.05), to which apoptosis (percent and MFI) was consequently positively correlated (p < 0.001 for percent and p < 0.05 for MFI). For telomerase, the highest positive predictive value (PPV) for mortality (93.3%) was at a cut-off value of 13 amol/ml, while those for apoptosis (85% for percent of apoptotic cells and 90.9% for MFI) were at a cut-off of 8% and 0.19 MFI. This makes the measurement of telomerase activity in ALL patients a potential tool to predict disease with unfavorable outcome and a candidate tumor marker.