Survivors of childhood acute lymphoblastic leukemia (ALL) treated with cranial radiation therapy (CRT) are at risk for cognitive impairment, although whether impairment progresses with age into adulthood is unknown. We report change in intelligence for 102 adult survivors of childhood ALL (age range, 26.6-54.7 years) during a median interval of 28.5 years. Survivors demonstrated lower Performance intelligence (mean, 95.3; standard deviation, 16.5; P = .005) but not Verbal IQ (mean, 97.4; standard deviation, 15.44; P = .09) at initial testing. Verbal intelligence declined an average of 10.3 points (P < .0001) during the follow-up interval with no decline in Performance intelligence. Decline was associated with current attention problems (P = .002) but not gender, CRT dose, age at CRT exposure, or years between testing. Results suggest long-term survivors of childhood ALL treated with CRT are at risk for progressive decline in verbal intellect, which may be driven by attention deficits. This trial was registered at clinicaltrials.gov as no. NCT00760656.