We examine the resource implications of two potential health services aimed at preventing congenital toxoplasmosis: a screening service involving serological surveillance for toxoplasma infection in pregnant women and its prophylactic treatment; and a health education campaign to help pregnant women avoid acquiring toxoplasma infection, through advice about special precautions regarding hygiene. Measuring the costs and benefits of prevention is complicated by uncertainty of the incidence of congenital toxoplasmosis in the United Kingdom, the extent of its harmful manifestations--mental and visual handicap--and the effectiveness of preventive measures. Hence an important aspect of this study is that it exemplifies the use of 'sensitivity analysis' as an aid to evaluation in the absence of hard data. We find that a screening service would seem unlikely to save resources, but that a health education campaign would seem much more likely to do so and further assessment of its potential effectiveness would be valuable.