OBJECTIVE: To determine the risk factors--especially the effects of thrombocytosis--for permanent visual loss in patients with temporal arteritis. METHODS: One hundred seventy-four patients with temporal arteritis (147 biopsy proven) were prospectively observed for the development of permanent visual loss. We used multivariate logistic regression analysis to determine which of 17 pretreatment characteristics were associated with visual loss. RESULTS: Visual ischemic manifestations occurred in 48 (28%) patients, including permanent visual loss in 23 (13%) patients. The independent predictors associated with an increased risk of permanent visual loss were a history of transient visual ischemic symptoms (odds ratio [OR] = 6.3; 95% confidence interval [CI]: 1.4 to 29; P = 0.02) and a higher platelet count (OR = 3.7 per SD; 95% CI: 1.8 to 7.9; P = 0.001). The presence of constitutional symptoms (OR = 0.14; 95% CI: 0.02 to 0.77, P = 0.01), polymyalgia rheumatica (OR = 0.04; 95% CI: 0.01 to 0.48, P = 0.02), and C-reactive protein level (OR = 0.35 per SD; 95% CI: 0.13 to 0.92, P = 0.03) were associated with a reduced risk. Upper limb artery involvement was excluded from the multivariate model, as no patients with that problem developed permanent visual loss. Of the 87 patients who presented with thrombocytosis (platelet count >400 x 10(9)/L), 32 (37%) developed ischemic visual symptoms, compared with 16 (18%) of those without thrombocytosis. CONCLUSIONS: An elevated platelet count is a risk factor for permanent visual loss in temporal arteritis. The finding of thrombocytosis in a patient with suspected temporal arteritis should emphasize the need for urgent treatment, with consideration of using inhibitors of platelet aggregation or anticoagulation therapy.