The relationship between ocular toxoplasmosis and levels of toxoplasma specific antibodies was examined in 195 patients. Using clinical information collected by questionnaires, patients were divided into: 97 with ocular toxoplasmosis (group 1) and 98 with ocular lesions not due to toxoplasma (group 2). The geometric mean of dye test titres (+/-S.D. natural log titre) in group 1 was 53.2 (+/-0.95) compared with 24.6 (+/-1.11) in group 2 (P < 0.001). Young females tended to have more active lesions compared with young males (P < 0.05). There was an age-dependent difference in dye test titres between the groups (P < 0.001). Group 1 showed a decline in titre with age compared with an increase in group 2. Ocular toxoplasmosis was diagnosed most frequently among 21-30 year olds. More group 1 patients had dye test titres > or = 65 iu/ml than group 2 (P < 0.05). Dye test titres > or = 65 iu/ml support a diagnosis of ocular toxoplasmosis whereas lower titres suggest other causes for eye lesions.