Granular corneal dystrophy Groenouw type I (CDGG1) is an autosomal dominant disease with complete penetrance. 124 blood samples were collected from a single Danish pedigree of seven generations. Linkage was discovered with markers on chromosome 5q, with IL9 (Z = 15.96; theta M = 0.027, theta F = 0.00) and D5S436 (Z = 11.75; theta M = 0.00, theta F = 0.081) flanking the disease locus most closely. The marker IL9 is located in the region 5q22-q32. By multilocus linkage analysis the most likely position of CDGG1 among 9 markers was: D5S396-IL9-CDGG1-D5S436-D5S210/D5S207++ +-D5S434-D5S119-D5S211 and CDGG1-D5S402-D5S434. In each of two independent small pedigrees, in which a milder form of CDGG1 occurs, the disease gene was also linked to IL9 (Z = 3.02 at theta = 0.0 in males and females); i.e. the severe and the milder forms may be allelic.