Neuropsychological outcome was evaluated in a prospective, longitudinal follow-up study of children age 4 months to 7 years at injury with either mild-to-moderate (N = 35) or severe (N = 44) traumatic brain injury (TBI). Age-appropriate tests were administered at baseline, 6 months, 12 months, and 24 months after the injury. Performance was compared on (1) composite IQ and motor, (2) receptive and expressive language, and (3) Verbal and Perceptual–Performance IQ scores. In comparison to mild-to-moderate TBI, severe TBI in infants and preschoolers produced deficits in all areas. Interactions between task and severity of injury were obtained. Motor scores were lower than IQ scores, particularly after severe TBI. Both receptive and expressive scores were reduced following severe TBI. Expressive language scores were lower than receptive language scores for children sustaining mild-to-moderate TBI. While severe TBI lowered both Verbal and Perceptual– Performance IQ scores, Verbal IQ scores were significantly lower than Perceptual–Performance IQ scores after mild-to-moderate TBI. Mild injuries may produce subtle linguistic changes adversely impacting estimates of Verbal IQ and expressive language. Within the limited age range evaluated within this study, age at injury was unrelated to test scores: The impact of TBI was comparable in children ages 4 to 41 months versus 42 to 72 months at the time of injury. All neuropsychological scores improved significantly from baseline to the 6-month follow-up. However, no further change in scores was observed from 6 to 24 months after the injury. The persistent deficits and lack of catch-up over time suggest a reduction in the rate of acquisition of new skills after severe TBI. Methodological issues in longitudinal studies of young children were discussed. (JINS, 1997, 3, 581–591.)