To investigate the changes of the activity of telomerase in peripheral blood mononuclear cells (PBMCs) from senile patients with pneumonia, the telomerase activity was examined before and after the stimulation of phytohemagglutinin-M (PHA-M) in PBMCs from 10 control subjects (group A), 12 non-senile patients with pneumonia (group B) and 9 senile patients with pneumonia (group C). Also observed was the proliferative response of these PBMCs to PHA-M. The results showed that, both with or without the stimulation of PHA-M, the values of telomerase activity in PBMCs from group C patients (A values: pre-stimulation, 0.43±0.04; post-stimulation, 0.63±0.03) were significantly lower than those in PBMCs from both group A patients (A values: prestimulation, 0.65±0.05; post-stimulation, 1.26±0.13;P<0.001, respectively) and group B patients (A values: pre-stimulation, 0.63±0.03; post-stimulation, 0.93±0.03;P<0.05, respectively). The results of MTT test showed that the proliferative activity of PBMCs in group C patients (A value: 0.35±0.03) was also significantly lower than that in group A patients (A value: 0.55±0.04;P<0.05) and group B patients (A value: 0.46±0.03;P<0.05). These results indicate that the telomerase activity decreases in senile patients with pneumonia, which may be one of the mechanisms for the weakened immune function in those patients.