BACKGROUND:Cognitive complaints are common in cancer survivors. We aimed to assess cognitive complaints in cancer survivors and the associated factors using a large web-based survey. METHODS: This online survey was proposed to cancer survivors. Participants completed several questions on cognitive complaints experience, expectations for support of cognitive difficulties, preexisting knowledge about chemotherapy-associated cognitive problems and demographic and medical variables. We used multivariable logistic regression models to estimate Odds Ratios and 95% confidence intervals to estimate associations. RESULTS: Among 1610 eligible participants (median age 52 [21-84]), >85% (n = 1393) were breast cancer survivors. Median postcancer treatment time (excluding hormone therapy) was 2.83 years [0.8-33]. Seventy five percent of the participants (n = 1214) reported cognitive complaints related to cancer treatments. Cognitive difficulties had an impact on work resumption for 76% of the participants (n = 754/982). Most cancer survivors would like to receive support (75%, n = 909) and especially cognitive training (72%, n = 658). Chemotherapy was strongly associated with cognitive complaints (multivariable OR = 3.67, 95% CI: 2.80-4.82). Self-reported sleep difficulties (ORoften vs. never = 2.84, 95% CI: 1.80-4.47), preexisting knowledge about chemotherapy-associated cognitive problems (ORNo vs. Yes = 1.69, 95% CI: 1-29-2.22) and age (OR21-64 vs. ≥65 = 0.37, 95% CI: 0.23-0.58) were also associated with cancer-related cognitive complaints. CONCLUSIONS: According to this large web-based survey including mainly breast cancer survivors, cognitive complaints were reported by three quarters of participants, which reinforces that cognitive difficulties are a major issue in cancer survivors. Chemotherapy, self-reported sleep difficulties and preexisting knowledge about chemotherapy-associated cognitive problems were strongly associated with cancer-related cognitive complaints. Most cancer survivors wished to receive support and especially cognitive training.