Objective To investigate the distribution characteristics of pathogenic bacteria, changes in the T-lymphocyte (T-cell) subsets and prognosis in the elderly patients with acute ischemic stroke complicated with severe pneumonia (SP). Methods A total of 150 elderly patients with acute ischemic stroke complicated with pneumonia at the Emergency Center of the First Affiliated Hospital of Xinjiang Medical University from March 2021 to March 2023 were enrolled as the study subjects. According to the severity of pneumonia, the patients were divided into the SP group and the non-SP (NSP) group. Clinical data, pathogenic bacteria culture and drug sensitivity test results, proportions of T-cell subsets in peripheral blood and 28-d mortality rate of patients were collected. SPSS 22.0 was used for statistical analysis. Data comparison between two groups was performed using t test orχ² test depending on data type. Results The mean age, National Institutes of Health Stroke Scale (NIHSS) score, prevalence of chronic obstructive pulmonary disease (COPD) and incidence of consciousness disorder in the SP group were higher than those in the NSP group, and the differences were statistically significant (P<0.05). The difference in the stroke site between the two groups was statistically significant (P<0.05). In 70 SP patients, 68 strains of pathogenic bacteria were cultured, including 60 (88.24%) Gram-negative bacteria and 8 (11.76%) Gram-positive bacteria. The three pathogenic bacteria with the top three detection rates were Acinetobacter baumannii (47.06%, 32/68), Pseudomonas aeruginosa (14.71%, 10/68) and Klebsiella pneumoniae (13.24%, 9/68). Acinetobacter baumannii was relatively sensitive to cotrimoxazole and cefoperazone/sulbactam. Pseudomonas aeruginosa was highly sensitive to tobramycin and amikacin and relatively sensitive to gentamicin and quinolones. Klebsiella pneumoniae was most sensitive to amikacin and imipenem. The levels of CD3⁺, CD4⁺ and CD4⁺/CD8⁺ in the SP group were lower than those in the NSP group, and the CD8⁺ was higher than that in NSP group, with statistically significant differences (P<0.05). The 28-d mortality rate in the SP group was 45.71% (32/70), which was higher than 6.25% (5/80) in NSP group (χ²=31.290; P<0.05). The levels of CD3⁺, CD4⁺ and CD4⁺/CD8⁺ in the deceased were lower than those in the survivors in the NSP group; CD8⁺ level was higher than that in the survivors; the differences were statistically significant (P<0.05). Conclusion Ischemic stroke with SP will increase the risk of death in patients. Age, NIHSS score, complication of CODP and disturbance of consciousness may be factors contributing to the progression of general pneumonia to SP in ischemic stroke. In addition, immune imbalance may also be involved in the mechanism of SP occurrence in ischemic stroke and increase the risk of death in patients.