Primary open angle glaucoma (POAG) is a genetically heterogeneous disease resulting in optic disc cupping and visual impairment. It can be inherited as either a complex or a monogenic trait. Autosomal dominant POAG is the most frequent type of monogenic glaucoma. In this study, we investigated the role of myocilin MYOC in Spanish patients with autosomal dominant POAG.We retrospectively analyzed the MYOC gene by PCR-DNA sequencing in five Southeast Spanish families and one Colombian family of Hispanic origin affected by autosomal dominant juvenile-onset open angle glaucoma (JOAG). We also analyzed two families with adult-onset POAG (AOAG).MYOC mutations D380A and P370L segregated with the disease in the five JOAG Spanish families and the Colombian family, respectively. Neither MYOC mutations nor cytochrome P4501B1 CYP1B1 mutations were detected in the AOAG families. The disease showed an insidious onset in D380A carriers, making early diagnosis difficult. A delay in diagnosis resulted in severe visual impairment. Topical medications were effective in controlling intraocular pressure (IOP) in D380A carriers, but 72.2% of them required surgery for long-term IOP control. Conversely, only 30% of AOAG patients required surgery. Mutation P370L was associated with a severe phenotype unresponsive to medical treatment. Analysis of the four MYOC-linked polymorphic microsatellite markers in the JOAG Spanish families revealed a common disease haplotype, indicating that the D380A mutation was inherited from the same founder.This is the first evidence of a founder effect for a MYOC mutation in Spanish JOAG patients. Analysis of the MYOC gene in Spanish patients with JOAG is useful to identify at-risk individuals thus help prevent visual impairment through early treatment.