Cancer-related fatigue (CRF) is a distressing late effect in childhood cancer survivors (CCS) with prevalence between 10-85% and little evidence on its risk factors. We aimed to describe the prevalence of CRF in adult CCS and assess its risk factors. As part of the CardioOnco study, we invited adult 5-year CCS treated at Inselspital Bern between 1976-2015 to a cardiooncological outpatient clinic and sent them questionnaires. We assessed fatigue with the Checklist Individual Strength subjective fatigue subscore (CIS, during last 2 weeks) and the Visual Analog Scale (VAS, at the current day). Increased fatigue was defined as CIS score 27-35 and VAS score ≥70. We collected information on previous cancer treatment and medical history and calculated mean CRF scores with ANCOVA adjusting for sex and age. We included 158 CCS (participation rate 29%) with median age at study of 33 years (IQR: 26-38). We found that 19% of CCS had increased fatigue with CIS and 11% with VAS. Mean CIS fatigue score was higher in women (21, CI 20-22) than men (18, CI 16-19, p = 0.001), in those treated with radiotherapy (22, CI 20-23 vs. 18, CI 17-19, p<0.001), those with sleep disturbance (23, CI 21-24 vs. 18, CI 17-19, p<0.001), and those with an endocrine abnormality (24, CI 22-25 vs. 18, CI 17-19, p<0.001). We found that one fifth of adult CCS experiences increased fatigue. Female CCS with history of radiotherapy and suffering from endocrine or sleep problems would profit from screening for CRF and further counselling with a specialist.