The 'GIST' score was developed to facilitate linkage analysis of adult-onset primary open angle glaucoma (POAG) pedigrees in the Glaucoma Inheritance Study of Tasmania (GIST). Previous genetic linkage studies on juvenile open angle glaucoma pedigrees have relied upon an analysis of definitely affected individuals using the 'single best diagnosis' convention. Studies of adult-onset POAG have been complicated by limited numbers of unequivocally affected members identified even in very large pedigrees due to the later onset of the disease. Many members of the pedigree may have equivocal clinical features or are too young to show signs of the disease. The 'GIST score' is a numeric value between zero and one where zero is clinical certainty of absence of the disease and one is the definitive diagnosis of POAG. The score is developed by assigning relative weighting to key clinical features which results in a 'pedigee probability' of the diagnosis being present or absent in a member of a pedigree. Ranking of borderline and unaffected glaucoma subjects allows the laboratory more flexibility in the use of the members of the pedigree for linkage analysis. The score is not intended to have clinical usefulness in management of glaucoma.