To elucidate the biological significance of dysplastic cells in malformations of cortical development, an immunohistochemical study was performed to investigate fibroblast growth factor-2 (FGF-2) expression in corticectomy specimens from epilepsy patients, including focal cortical dysplasia (FCD) with balloon cells (BCs) (n = 4; age/sex = 2M, 14F, 24M, 45M), tubers of tuberous sclerosis complex (TSC-tubers) (n = 2; 1F, 3F), FCD without BCs (n = 3; 23F, 23M, 25M), and gliotic lesions (n = 3; 12M, 25M, 29M). The nucleus and/or cytoplasm of astrocytes in all cases examined were positive for FGF-2; however, FGF-2 immunoreactivity was not detected in oligodendroglial cells. In all dysplastic lesions, FGF-2 was detected in the astrocytic nuclei, and cytoplasm and/or nuclei of BCs. Dysplastic neurons (DNs) in FCD with BCs and TSC-tubers were variably positive for FGF-2 in the cytoplasm, but FGF-2 was not detected in the neurons of FCD without BCs. The number of FGF-2 immunoreactive cells (FGF-2-IR%) in FCD with BCs (46.0 ± 4.1%) was higher than that in FCD without BCs (19.8 ± 3.1%) and gliotic lesions (19.5 ± 3.3%) with statistical significance (P < 0.001). These results, together with previous reports showing FGF-2 expression in neuroblasts and glioblasts in human fetal brain, and mainly in astrocytes in adult brain, suggest that FGF-2 expression in MCDs reflects incomplete differentiation and maturation of dysplastic cells, and that FGF-2-IR% is associated with histological subtypes of MCD, reflecting the timing of insults underlying the pathogenesis of each disorder.