The study examined the association between lymphocyte subsets (total lymphocytes, CD3+, CD4+, CD8+, B cells, NK cells) and clinical outcomes in 959 patients hospitalized with COVID-19 from January 2020 to April 2021. Blood samples were collected within the first 24 hours of admission and analyzed using immune phenotyping. Of the patients, 29.4% required ventilatory support, 11.3% were admitted to the ICU, and 10.7% died in the hospital. In multivariable logistic regression adjusted for clinical factors and inflammatory markers, low CD3+ cells (cut-off value 666 cells/mm³) were independently associated with the need for ventilatory support (aOR: 2.3, 95% CI: 1.5–3.4, p=0.013) and in-hospital death (aOR: 2.4, 95% CI: 1.0–5.8). Low CD4+ cells (cutoff 359 cells/mm³) were independently associated with in-hospital death (aOR: 2.8, 95% CI: 1.4–5.5, p=0.045). Adaptive immunity, especially CD3+ and CD4+ T cells, is important for the prognosis of patients at high risk of a severe course of COVID-19. The results may help identify patients at risk of needing ventilatory support and dying in hospital.