Routine fluoroquinolone (FQ) prophylaxis may increase the risk of antimicrobial resistance, microbiome disruption, and Clostridioides difficile infection in patients receiving chimeric antigen receptor T-cell (CAR-T) therapy for hematological malignancy. In Australia, FQ prophylaxis is not routinely used. We evaluated the etiology of early fever following CAR-T to better understand the incidence of infections, particularly bloodstream infections, in a cohort not receiving FQ prophylaxis. This bicentric Australian retrospective study included adults receiving standard-of-care CD19 CAR-T therapy for DLBCL (2019 to 2023). The primary outcome was the cause of sustained fever (≥38.0°C on ≥1 days) from infusion to day 30. Recurrent fever required ≥72 h afebrile before a new fever. Infections were classified as microbiologically confirmed, clinically defined, or fever syndrome per consensus criteria. About 204 adults (median age 64 years, IQR: 57 to 71) received tisagenlecleucel (50%) and axicabtagene (50%), after a median of 3 prior therapies (interquartile range [IQR]: 3 to 4). Sustained fever occurred in 131/204 patients (64%), comprising 161 episodes. Of these, 36 (21%, 28pts) were microbiologically confirmed infections, 14 (9%, 14pts) were clinically defined infections, and 110 (69%, 108pts) were fevers of unknown origin. Bacteremia occurred in 7/204 patients (3.4%; 9 events), with one fatal polymicrobial bacteremia. Other microbiologically confirmed infections included C. difficile (7/36), URTI (13/36) and invasive fungal infection (5/36). Risk factors for early microbiologically confirmed infection in univariate analysis included axicabtagene product (hazard ratio [HR] = 2.5, P = .019), grade ≥3 immune-cell associated neurotoxicity (HR = 3.4, P = .012), and prolonged neutropenia (absolute neutrophil count ≤ .5 × 10⁹/L for ≥14 days; HR = 3.7, P = .014). Early bacteremia rates remain low without routine FQ prophylaxis. Initial sustained fevers are predominantly noninfectious. Our data do not support universal FQ prophylaxis in CAR-T therapy.