Objective: To determine the incidence of acute symptomatic toxoplasma retinochoroiditis presenting to ophthalmologists for patients born in Britain and elsewhere. Design: Population based, cross sectional study. Setting: 11 districts in south Greater London. Subjects: All patients presenting to NHS ophthalmologists with symptoms due to acute toxoplasma retinochoroiditis in 1992-3. Main outcome measure: Intraocular inflammation in association with a retinochoroidal scar, active adjoining retinitis, and IgG serum antibodies to toxoplasma. Results: The estimated incidence of acute symptomatic retinochoroiditis for all people born in Britain was 0.4/100 000/year and for black people born in west Africa 57/100 000/year. If a mean of two symptomatic episodes per lifetime is assumed, 100 people born in Britain may be affected each year, about a fifth of the estimated 500-600 congenitally infected people born each year. Conclusions: A substantial proportion of people with acute symptomatic toxoplasma retinochoroiditis were born outside the country, and the number born in Britain was smaller than the number previously estimated to develop retinochoroidal lesions due to congenital toxoplasmosis. These findings suggest that prenatal screening for toxoplasmosis in Britain may be of limited benefit. Key messages Prevention of toxoplasma retinochoroiditis is considered to be a major benefit of prenatal screening as previous studies have suggested that 80% of congenitally infected children would eventually develop retinochoroiditis This report shows that the incidence of symptomatic toxoplasma retinochoroiditis is nearly 100 times higher in black people born in west Africa than in subjects born in Britain In subjects born in Britain acute symptomatic toxoplasma retinochoroiditis is much less common than expected, affecting about one fifth of the estimated number of congenitally infected people These findings suggest that the potential benefits of prenatal screening for toxoplasma infection in Britain are limited.