Type 1B astrocytes of the human optic nerve head (ONH) constitutively express neural cell adhesion molecule (NCAM) in vivo and in vitro. Increased synthesis of NCAM has been detected in reactive astrocytes in the glaucomatous ONH of human donor eyes. Several NCAM isoforms are generated through alternate RNA splicing in tissue- and disease-specific patterns. In this study, we analyzed expression of NCAM isoforms in ONH of normal donors at different ages and in glaucoma. Total RNA was extracted from ONH of fetal, normal adult and glaucomatous eyes, and cultured human ONH astrocytes, fetal brain astrocytes and an astrocytoma cell line, for reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. To distinguish between NCAM 180 and 140 isoforms, exon-specific primer sets covering exons 13-19 were used. Isoform-specific riboprobes were used for in situ hybridization (ISH) in glaucomatous and in age-matched ONH. By RT-PCR, NCAM 140 was the predominant isoform in adult ONH as well as in all cultured cells. NCAM 180 mRNA was strongly expressed in glaucoma, whereas in normal adult tissues it was not detectable. ISH confirmed expression of NCAM in normal adult ONH and localized NCAM 140 mRNA to astrocytes. ISH demonstrated expression of NCAM 180 mRNA in reactive astrocytes in glaucomatous ONH. Our results demonstrate that the NCAM 180 isoform is induced in glaucoma. NCAM 180 may play a role in astrocyte interaction with extracellular matrix (ECM), vessels, axons and other astrocytes and, through its expanded cytoplasmic domain, serve as a signaling molecule for reactive astrocytes during remodeling of the ONH in glaucoma.