Immunity to a major corneal antigen was studied in 28 Fuchs' heterochromic cyclitis patients and compared with the response of 44 patients with other types of uveitis and 19 healthy controls. The highest incidence of immune response was found in patients with anterior segment involvement only (anterior uveitis and Fuchs') whereas the frequency of anti-corneal immune response in patients with posterior segment involvement only was low and not significantly different from that of healthy controls. Cellular immunity to corneal antigens was found in the majority of Fuchs' heterochromic cyclitis patients, and in one-third of the anterior uveitis patients. No correlation could be established in these patients between a positive cellular response and the chronicity of the disease or the presence of keratic precipitates. Humoral immunity to the corneal antigen was also the highest in patients with anterior segment involvement, but there was no difference in response between Fuchs' and non-Fuchs' anterior uveitis patients. This study suggests that anti-corneal immunity may be triggered in inflammatory diseases of the anterior segment, especially in Fuchs' heterochromic cyclitis.