This paper makes use of unique matched birth-school records from all children born in Florida between 1994 and 2002 to study the effects of having a severely disabled sibling on children's academic achievement. We follow the over 140,000 first-born children who have a sibling born during this time period, and observe the effects of having a disabled sibling on children's test scores and behavior. We find that the more severe the disability, the larger the negative consequences are on the older sibling, and that this effect is strongest the closer together the siblings are in age. (We also conduct analyses looking at the effect of a disabled twin on a healthy twin and find even larger effects.) The estimated effects differ substantially by maternal education and family income.