Studies are emerging that suggest that major language indices do not differentiate children treated for acute lymphoblastic leukemia (ALL) with risk-adapted intrathecal chemotherapy (ITC) from control children matched on age, gender, and educational level. No study to date has controlled for cognitive environment, an important variable influencing language achievement and outcome. This case-control study applies the deconfounding principle by using a sibling as a control to investigate language outcomes in a male child 11 years after administration of ITC for ALL at the age of 2 years 3 months. A comprehensive behavioral language test battery failed to differentiate the siblings on current language performance when descriptively compared, but neurophysiological assessment revealed that the ITC-treated child required more time and elicited a smaller N400 component compared to his sibling during picture-word matching. The findings suggest that in the absence of pretreatment performance indices, comparison with sibling achievement may supplement what is known on posttreatment language skill development drawn from comparative studies using children matched on age, sex, and educational level drawn from the community. The study's findings offer pilot data of language outcomes following ITC beyond the early stage of survivorship. The benefits and limitations of using siblings in research where the cognitive environment is known to make an important contribution to skill development are discussed.