In an observational study, the incidence of infectious events was monitored in 133 patients with acute lymphoblastic leukaemia, chronic lymphocytic leukaemia, or non-Hodgkin lymphoma treated with anti-CD19 CAR T cells in a phase I/II study. Within 28 days after infusion, 23% of patients had infections. The infection density was 1.19 infections per 100 days at risk in this initial period of time and 0.67 infections per 100 days between 29–90 days after infusion. Invasive fungal infections occurred in 5% of the patients, and life-threatening or fatal infections in 4% of them. The incidence, distribution, and severity of infections observed in this study are similar to those observed in clinical trials of salvage or primary chemotherapy for patients with the same malignacies. Therefore, strategies established in other settings to prevent infections in immunocompromised patients could be adapted to improve outcomes of CAR-T-cell therapy.