Background: The presence of health problems in a child is known to be negatively associated with later academic achievement, but less is known about the educational outcomes for siblings of children in poor health. The study investigated how having a sibling with health problems affects a healthy sibling's academic achievement. Methods: We utilized medical and social microdata from Swedish administrative population registers. Our sample consisted of N = 115,106 individuals (51.3% boys) born in 1990 in Sweden. We compared children with ill siblings to children whose siblings did not have poor health. Siblings' hospital admissions and the academic achievements of the healthy sibling during their final year of compulsory education (at the age of 15-16) were analyzed using linear and logistic regression in relation to individual health-and family-related confounders. Results: Sibling hospitalization was significantly associated with lower overall grade points ([beta]= -10.73, p < 0.001) and an increased odds ratio (OR) of ineligibility for upper secondary education (OR = 1.42, 95% confidence interval = 1.31-1.52, p <0.001). Conclusions: School and health personnel should also consider the needs of healthy siblings during their work with children in poor health, because they too can be disadvantaged.