Despite impressive outcomes in selected patients, significant heterogeneity in clinical response to CAR-T cell therapy remains. The gut microbiome (GM) has recently emerged as one of the key modifiable factors of prognosis and response to treatment in cancer patients, with high-diversity profiles rich in health-associated taxa while poor in pathobionts generally associated with better response and longer survival. Currently, it is unknown if GM also modulates anti-tumor responses to CAR-T cells and related toxicities in lymphomas.