To determine the frequency and clinical characteristics of Uhthoff's symptom, I studied 100 consecutive patients with pregeniculate visual loss of diverse etiology. Eighteen patients reported Uhthoff's symptom. Although there was a significant association of Uhthoff's symptom with multiple sclerosis (10 patients), other etiologies for this symptom included tumor (3 patients), optic neuropathy or papillitis (3 patients), aneurysm, and orbital pseudotumor (1 patient each). There was no link between the severity of visual loss, as measured by diminished acuity, field loss, or binocular involvement, and Uhthoff's symptom.