A case-control study was conducted in a brucellosis low-prevalence area of NW Spain to determine factors associated with high within-flock small-ruminant brucellosis prevalence in 1998. Forty-one cases and 69 controls were selected and information from both official sources and personal interviews was retrieved for every flock. The relationship between variables obtained and flock status was assessed by unconditional multivariable logistic regression analysis. The introduction of replacement animals into the flock, the presence of older farmers, an inadequate brucellosis vaccination programme and higher flock seroprevalence in the town in 1997 were positively associated with case flocks. Thus, specific actions directed at farms presenting these characteristics should be included within official eradication programmes. In addition, for the 1999 campaign the time from sampling to culling the seropositive animals correlated positively (r=0·53; P<0·01) with the flock seroprevalence the following year, suggesting the need for a faster removal of the infected animals to increase the efficacy of the eradication campaigns.