Serum samples from 9006 women, who delivered in Switzerland in 1990 and 1991, were collected around the country. Of these women, 62.7% were Swiss and 37.3% originated from foreign countries. Samples were first screened for anti-HBc and those found positive were further tested for HBsAg, anti-HBs and anti-HDV. Anti-HBc was found in 640 of the 9006 women (overall prevalence, 7.1%; Swiss, 3.3%; foreigners, 13.5%). Of these 640 positive samples, 61 (9.5%) were positive for HBsAg (without anti-HBs), 467 (73.0%) positive for anti-HBs (without HBsAg) and 8 (1.3%) positive for both HBsAg and anti-HBs. The remaining 104 were thus anti-HBc positive without HBsAg or anti-HBs. These 104 specimens with the so-called "isolated anti-HBc" reactivity represented 1.2% of the whole population or 16.3% of the 640 anti-HBc positive mothers. All were HBV DNA negative (PCR). Anti-HDV antibody was found in only five women. HBsAg was seen in 38 of the cord-blood samples from the anti-HBc positive mothers. In this large sampling, we observed a relatively high seroprevalence of HBV infection. Cases with isolated anti-HBc reactivity, being HBV DNA negative by PCR, were probably non-infectious at the time of blood collection.