To analyze the association between Fuchs' heterochromic iridocyclitis (FHI) and toxoplasmosis, we performed ocular examinations and used various specific laboratory tests to establish a role for Toxoplasma gondii in the pathogenesis of FHI. Results were compared with those for other types of uveitis and healthy controls. Of the 88 patients with FHI, nine (10.2%) had toxoplasmosislike scars, but an association could not be proved by the indirect immunofluorescence antibody test or enzyme-linked immunosorbent assay, or by a test for cellular immunity to Toxoplasma antigen. Analysis of aqueous humor samples for Toxoplasma antibodies in patients with FHI also yielded negative results. On the basis of the negative results of these laboratory tests, we concluded that FHI is not associated with ocular toxoplasmosis.