In a study of lambs from 194 sheep flocks from different areas of Norway, 44.3% of the flocks were found seropositive against Toxoplasma gondii using an ELISA test, with antibodies found in 16.2% of the 1940 individual animals. The risk factors for the occurrence of antibodies to Toxoplasma gondii found in the multivariate logistic regression were: daily presence of a young cat in the sheep house (Odds ratio, OR = 4.11, 95% CI = 1.01-19.7); 'atypical' grazing of lambs (OR = 6.35, CI = 2.36-17.11); use of mouse poison in the sheep house (OR = 2.26, CI = 1.02-5.00); farm situated at an altitude > 100 meters above seal level (101-250 m: OR = 1.20, CI = 0.49-2.92; 251-500 m: OR = 4.97; CI = 2.04-12.0; > 500 m: OR = 3.66, CI = 1.33-10.1). A lower risk was found for flocks with perforated metal floors in the sheep house (OR = 0.47, CI = 0.23-0.96) and timber construction of the sheep house (OR = 0.34, CI = 0.15-0.80). Based on these findings it was recommended that farmers avoid keeping young cats in the sheep houses, that close-to-farm grazing be kept to a minimum and that perforated metal floors be used in the sheep houses. However, with such a high seroprevalence, the proposed measures alone would not reduce the occurrence of Toxoplasma in lambs to a level where undercooked lamb can be consumed without posing an unacceptable risk for some consumer groups.