During a recent foot-and-mouth disease epidemic in Argentina, cattle herds affected in 2001 were located mainly (69%) in Buenos Aires province. The densities of outbreaks (no. of outbreaks per km2) and cattle-demographic variables in the province were estimated using a geographical information system and kernel function. Before the epidemic officially was recognized, the density of outbreaks was correlated (rsp = 0.28-0.47) with the geographic distribution of small (< or =100 cattle), dairy and fattening herds. During the mass-vaccination campaign to control the epidemic (April-July), the density of outbreaks was most strongly correlated (rsp = 0.20-0.25) with the distribution of large (>500 cattle) and breeding herds. After the end of the mass-vaccination campaign, large herds and number of cows were most strongly correlated (rsp = 0.16-0.26) with outbreak density. These relationships might indicate that: (1) the disease spread more rapidly or was more easily detected in intensive production systems at the beginning of the epidemic; (2) vaccination and other control methods applied were less effective in large, semi-intensive production systems; (3) incomplete vaccine protection was responsible for herd outbreaks that occurred after the end of the mass-vaccination campaign.