The aim of this study was to prospectively investigate the prevalence of and risk factors for psychosocial problems in siblings of paediatric cancer patients. One and 6 months after diagnosis, sibling self-reported anxiety, social-emotional problems and quality of life (QoL) were assessed, as were the predictor variables: sibling prediagnosis functioning, age and gender and the ill child's diagnosis. At 1 month, siblings reported a lower QoL and adolescent girls reported more emotional problems compared with peers. At 6 months, adolescent QoL remained relatively impaired. Over time, adolescent brothers reported fewer emotional and total problems and young girls reported decreased anxiety. No significant amelioration in QoL was found over time. The older the siblings were, the lower their observed QoL at both measurements and in several domains. The occurrence of life events predicted sisters' QoL at 1 month. Changes in sibling functioning were predicted by none of the investigated risk factors. Thus, QoL is impaired shortly after diagnosis. Adolescent siblings risk persisting problems in daily functioning. Further prospective research on other risk factors such as coping and family functioning over time is needed.