Background: [ 18F]fluorodeoxyglucose-positron emission tomography (PET) is emerging as a strong diagnostic and prognostic tool in follicular lymphoma (FL) patients. Patients and methods: In a subset analysis of the FOLL05 trial (NCT00774826), we investigated the prognostic role of post-induction PET (PI-PET) scan. Patients were eligible to this study if they had a PI-PET scan carried out within 3 months from the end of induction immunochemotherapy. Progression-free survival (PFS) was the primary study end point. Results: A total of 202 patients were eligible and analysed for this study. The median age was 55 years (range 33–75). Overall, PI-PET was defined as positive in 49 (24%) patients. Conventional response assessment with CT scan was substantially modified by PET: 15% (22/145) of patients considered as having a complete response (CR) after CT were considered as having partial response (PR) after PI-PET and 53% (30/57) patients considered as having a PR after CT were considered as a CR after PI-PET. With a median follow-up of 34 months, the 3-year PFS was 66% and 35%, respectively, for patients with negative and positive PI-PET (P < 0.001). At multivariate analysis, PI-PET (hazard ratio 2.57, 95% confidence interval 1.52–4.34, P < 0.001) was independent of conventional response, FLIPI and treatment arm. Also, the prognostic role of PI-PET was maintained within each FLIPI risk group. Conclusions: In FL patients, PI-PET substantially modifies response assessment and is strongly predictive for the risk of progression. PET should be considered in further updates of response criteria. Key words: FDG-PET, follicular lymphoma, prognosis