PurposeAsymptomatic patients with follicular lymphoma (FL) and a low tumour burden can be followed without initial therapy, a strategy called watchful waiting (WW). Prediction of the time to treatment (TTT) is still a challenge. We investigated the prognostic value of baseline total metabolic tumour volume (TMTV) and whole-body total lesion glycolysis (WB-TLG) to predict TTT in patients with FL on WW.MethodsWe conducted a retrospective study of 54 patients with FL (grade 1–3a) diagnosed between June 2013 and December 2019, staged with FDG PET/CT, and managed on WW. Median age was 62 years (range 34–85), stage was advanced (III–IV) in 57%, and FLIPI score was intermediate to high (≥ 2) in 52% of the patients.ResultsThe median TMTV and WB-TLG were 7.1 and 43.3, respectively. With a median follow-up of 59 months, 41% of patients started immuno-chemotherapy. The optimal cut-points to identify patients with TTT within 24 months were 14 for TMTV (AUC 0.70; 95% CI 51–88) and 64 for WB-TLG (AUC 0.71; 95% CI 52–89) (p< 0.005). The probability of not having started treatment within 24 months was 87% for TMTV < 14 and 53% for TMTV ≥ 14 (p< 0.005). TMTV was independent of the FLIPI score for TTT prediction. Patients with both FLIPI ≥ 2 and TMTV ≥ 14 had only an 18% probability of not having started treatment at 36 months, while this probability was 75% in patients with TMTV < 14.ConclusionMetabolic tumour volume parameters may add information to clinical scores to better predict TTT and better stratify patients for interventional studies.