Bulk milk samples from 2236 dairy herds randomly selected throughout Sweden in proportion to region and herd size were analysed for antibodies to bovine coronavirus (BCV) in an ELISA. The results were expressed as optical density (OD) values and an OD>0.04 was considered positive. Eighty-nine per cent of the samples were antibody-positive and 52 per cent had high levels of antibodies to BCV (an OD>0.70). There were significantly higher OD values (P<0.001) and fewer antibody-negative samples (P<0.001) from larger herds than from smaller herds. There were also significantly higher OD values and fewer antibody-negative samples from herds in southern Sweden than from herds in northern Sweden (P<0.001 and P<0.001, respectively). These results indicate a higher frequency of BCV infections in larger herds and in herds in southern Sweden.