The GLC1A locus for autosomal dominant juvenile and middle age onset primary open angle glaucoma (OAG) has been mapped to chromosome 1q21-q31. OAG, however, is a heterogeneous disease. We tested linkage of OAG and ocular hypertension (OHT), a major risk factor for OAG, to GLC1A in eight French families with multiple cases of juvenile and middle age onset OAG. There was strong evidence of genetic heterogeneity, four families being linked to GLC1A and two or three others being unlinked, depending on whether the complete OAG phenotype was analysed alone or jointly with OHT. Peak intraocular pressure (IOP) did not differ significantly between the two groups of families, while linkage to GLC1A conferred a highly increased risk of developing OAG and of having severe glaucomatous optic neuropathy. Testing linkage of familial OAG to GLC1A may therefore have prognostic value too.