Balloon cells (BC) are the prominent and defining cellular component of type IIB Focal Cortical Dysplasia (FCD), a common cause of focal epilepsy in patients undergoing surgical treatment. BC are considered immature cells of uncommitted cellular differentiation having immunophenotypical characteristics of both neurones and glia. They are often located in the lower cortical layers and white matter underlying the dysplastic cortex, suggesting migratory arrest during development. We investigated the proliferative potential of BC in 15 cases of FCD from patients with a wide range of ages using immunohistochemistry for Mcm2 (mini chromosome maintenance protein) and Ki67. In the majority of cases, BC showed Mcm2 nuclear positivity. In addition, cells with intermediate neuronal-glial characteristics were labelled whilst the dysmorphic or hypertrophic pyramidal neuronal components of FCD were not. Ki67 labelled only occasional BC. These findings support the view that BC cells represent a pool of less differentiated glial cells with proliferative capacity which may have potential for delayed neuronal differentiation. Furthermore, as Mcm2 specifically identifies BC populations, this marker may be of diagnostic value in the subtyping of FCD lesions in patients with epilepsy.