OBJECTIVE To investigate school absenteeism among childhood cancer survivors and their siblings and examine factors related to absenteeism in survivors. METHODS A cross-sectional study was conducted among consecutive cancer survivors attending a large pediatric cancer survivor clinic. Absenteeism rates were obtained for survivors and their closest in age sibling from school report cards. Absenteeism was compared with a population control group of 167752 students using 1-sample t tests. The Child Vulnerability Scale, Pediatric Quality of Life Inventory, and Behavior Assessment System for Children were administered to survivors. Univariate and multiple regression analyses assessed variables associated with days absent. RESULTS One hundred thirty-one survivors (median age at assessment: 13.4 years, range 8.0-19.2; median age at diagnosis: 9.4 years, range 4.3-17.3) and 77 siblings (median age at assessment: 13 years, age range 7-18) participated. Survivors and siblings missed significantly more school days than the population control group (mean ± SD: 9.6 ± 9.2 and 9.9 ± 9.8 vs 5.0 ± 5.6 days, respectively, P < .0001). Among matched survivor-sibling pairs (N = 77), there was no difference in absenteeism (9.6 ± 9.2 vs 9.9 ± 9.8 days, P = .85). Absenteeism in survivors was significantly associated with a low Pediatric Quality of Life Inventory Physical Health Summary Score (P = .01). Parents' perception of their child's vulnerability and emotional and social functioning were not associated with absenteeism. CONCLUSIONS Childhood cancer survivors and siblings miss more school than the general population. The only predictor of absenteeism in survivors is poor physical quality of health. More research should be devoted to school attendance and other outcomes in siblings of childhood cancer survivors.