During one year from June 1992 serum IgG antibodies to Toxoplasma gondii among 35,940 pregnant women were measured in a cross-sectional study conducted in Norway. The overall prevalence was 10.9%. The lowest prevalences were detected in the north (6.7%) and in the inland counties (8.2%). A significantly higher prevalence was detected in the southern counties (13.4%) where a mild, coastal climate prevails. Women with foreign names had a higher prevalence (22.6%) than women with Norwegian names (10.0%). The high prevalence among women living in the capital city (Oslo) as compared to other cities and rural areas (13.2% vs. 10.1% and 10.2% respectively), was explained by the higher proportion of foreign women in Oslo. Prevalence significantly increased with age in women over 34 years old. This increase was only detected among women with Norwegian names. An increase in prevalence according to number of children was detected. Women without children had a prevalence of 8.8% while women with three children or more had a prevalence of 14.9%. Multivariate analyses showed that being seropositive was independently associated with county of residence, age, nationality and number of children.