Data from two global studies (6-week open-label, phase 1 study; 20-day double-blind, phase 3 study) and their 1-year open-label extensions were pooled to assess long-term effects of adjunctive topiramate on adaptive behavior in infants with clinical or video-electroencephalographic evidence of refractory, partial-onset seizures. The primary safety and efficacy results of adjunctive topiramate treatment were reported previously. We report the changes in adaptive behavior of infants, based on Vineland Scales of Adaptive Behavior. Of 284 infants (mean [S.D.] age, 12 [6.3] months) enrolled, 89% (n = 252) manifested partial-onset seizures, and 41% (n = 116) manifested clinically relevant, symptomatic etiologies at pretreatment baseline. Overall, Vineland scores were below average at pretreatment baseline. The most frequently used concomitant antiepileptic drugs included valproic acid (59%), phenobarbital (31%), and carbamazepine (19%). The most common treatment-emergent cognitive and neuropsychiatric adverse events included anorexia (35%) and somnolence (27%). A clinically significant decline (approximately 15 points, or 1 S.D.) occurred in both Vineland Scales composite (mean change, −14.0) and domain standard scores from pretreatment baseline to open-label extension endpoint. However, individual domain raw scores increased, indicating that infants progressed in acquisitions of adaptive skill, but at a slower rate than the normative population.