Activation-induced cytidine deaminase (AID/AICDA) is required for somatic hypermutation and class-switch recombination of the immunoglobulin gene, and for c-myc translocation of germinal center-derived B-cell lymphoma. In the present study, we attempted to clarify the significance of AID associated with c-myc in the progression of follicular lymphoma (FL) using RT-PCR and quantitative real-time PCR. Tissues from the patients with grade 3 FL expressed relatively higher levels of c-myc and AID. The samples taken from a patient with FL who died within 2 years after the start of treatment showed either no or low expression of AID, despite expressing high levels of c-myc. In order to examine the role of AID expression in rapidly progressive FL, the full-length AID transcript was transfected into AID-negative cell lines established from different patients with rapidly progressive FL. This led to the establishment of AID-expressing transfectants with a low proliferation rate and a significantly increased incidence of G0/G1 arrest compared with controls. Our results indicate that AID may act as a negative regulator of cell survival in FL when sufficient c-myc is expressed. Switch-off or low expression of AID after c-myc amplification may correlate with the clinical outcomes of FL. (Cancer Sci 2012; 103: 415421)