Recent research has suggested that childhood onset of localization-related (focal) temporal lobe epilepsy is associated with a generalized adverse effect on cognition and brain structure, especially cerebral white matter volume. This study examined the neurodevelopmental impact of childhood onset epilepsy on corpus callosum volume and the cognitive consequences of reduced cerebral connectivity. Healthy controls (n = 15) and patients with temporal lobe epilepsy (n = 32) were matched on gender and handedness, and childhood and adult onset epilepsy groups were matched on duration of epilepsy (mean = 19 years) but varied in neurodevelopmental age at onset of recurrent seizures. Results showed that childhood onset of temporal lobe epilepsy was associated with significant volumetric reduction of the corpus callosum compared to both late onset and healthy controls, with the latter two groups not differing from one another. The volumetric loss was most evident in posterior followed by anterior corpus callosum. Volumetric reduction of the corpus callosum in temporal lobe epilepsy was of clinical significance with smaller volumes associated with poorer performance on measures of nonverbal problem solving, immediate memory, speeded complex psychomotor ability and fine motor dexterity. These findings indicate that childhood onset of temporal lobe epilepsy is associated with an adverse neurodevelopmental impact on brain connectivity which is of clinical consequence and theoretical interest.