The prevalence of hypothyroidism in British patients with primary open angle glaucoma (POAG) was examined. A recently reported study from Montreal had shown a significant increase (p < 0.004) in biochemical hypothyroidism (23.4%) in a population of 64 POAG patients compared with controls (4.7%). Mechanisms for a possible causal association between the two diseases are discussed, including mucopolysaccharide deposition in the trabecular meshwork and vasculopathy altering ocular bloodflow. Reports of improved glaucoma control following treatment of hypothyroidism are discussed. This study examined 100 consecutive patients with POAG in a specialist glaucoma clinic. All patients were questioned regarding symptoms of thyroid dysfunction and previous thyroid disease. All patients not already taking thyroxine underwent an assay of thyroid stimulating hormone. The 4% (95% CI 1.1-9.4%) prevalence of overt hypothyroidism in our study shows no clinically significant increase either over controls in the Montreal study or over our local population. We conclude that in our local population there is no evidence for a clinically important association of hypothyroidism with glaucoma.