Background The COVID-19 pandemic is a highly traumatic event that may lead to a greater risk of developing psychological disorders, especially in cancer patients who are more likely to be infected with the virus and to develop complications. The objective of this study was to measure anxiety levels among cancer patient during COVID-19 pandemic and the associated factors including patients’ conditions of lockdown. Go to: Methods A cross-sectional study was conducted among adult cancer patients (hematological and solid tumors) receiving outpatient treatment or during follow-up in a French Comprehensive Cancer Centre. A postal self-administered questionnaire was sent to 4000 patients in June 2020, including Anxiety (Stait Trait), Fear of a cancer recurrence (FCR) as well as questions relative to socio-demographics, management of cancer care during the pandemic and the conditions of lockdown. Go to: Results A total of 1097 patients completed the questionnaire (63.2% female; mean age 64.7 years ±12.3 years, 24.3% haematological cancers). Mean IES-R score was 15.7 ([0-81]) and 14.7% of patients had moderate or severe post-traumatic stress (score ≥33). Mean anxiety score was 39.0 (SD=13.6, range: [20-80]) with 30.5% of patients having anxiety symptoms. In the multivariate analysis we found that anxiety level was significantly increased for younger patients (OR=1.69, 95%IC [1.01-2.82]), female (OR=1.65, 95%IC [1.05-2.59]), patients with a high FCR score (OR=4.90, 95%IC [2.84-8.44]), patients unsatisfied with the current management of their cancer (OR=2.4, 95%IC [1.58-3.66]) and patients afraid of coming to hospital for fear of COVID (OR=2.10, 95%IC [1.32-3.35]). Protective factors against anxiety were staying busy during the lockdown period (OR=0.46, 95%IC [0.30-0.72]) and seeing the positive aspects of lockdown (OR=0.43, 95%IC [0.28-0.66]). Go to: Conclusions These results contribute to a better understanding of the psychological consequences of COVID-19 pandemic in the context of cancer and highlight the need to better support patients at high risk of developing high anxiety levels. Conditions of lockdown are important to contain anxiety among cancer patients.