Infection with reovirus serotype 3 (reo 3) has been postulated to be associated with extrahepatic biliary atresia (EHBA) in infants, and with neonatal hepatitis (NNH). We have investigated this association by assaying antireo 3 antibodies in sera from infants (aged < 4 months) with EHBA (n = 40), NNH (59), cholestatic liver disease due to other causes (61) and control infants with no liver disease (138). Antireo 3 immunoglobulins (Ig) of the G, A and M classes were measured by enzyme-linked immunosorbent assay. No differences in the prevalence of antireo 3 IgG or IgA were found between any of the four groups. A significantly higher prevalence of positive antireo 3 IgM was found in infants with EHBA (12/40), NNH (12/59) or cholestatic liver disease associated with parenteral nutrition (7/17), alpha-1 antitrypsin deficiency (4/15) or a variety of other causes (15/29) compared with control infants (13/138). These data support an association between reovirus 3 infection and cholestatic liver disease in infants. The nature of this association may differ for EHBA, NNH and cholestatic liver disease due to other causes, and remains to be determined.