Foot-and-mouth disease (FMD) is a highly contagious virus infection of sheep, goats, cattle, pigs and other, non-domesticated species of artiodactyls, and causes both clinical and sub-clinical infection according to the natural or acquired immunity of the host. Within vaccinated dairy herds FMD may appear as an acute, mild or subclinical infection, dependent upon the immune status of the herd, the level of challenge and the efficacy of the vaccine used. In the large dairy herds of Saudi Arabia, sub-clinical FMD was on a number of occasions, found to have spread amongst the cattle before signs of disease were seen. Such undetected transmission resulted in a large incidence on the first day of diagnosis and curtailed the impact of post- outbreak vaccination (PoV). First day incidence (FDI) for these herds was found to correlate with the final cumulative incidence of clinical disease. Since FDI is available at the start of an outbreak it can be used as a predictive tool for the eventual outcome of an FMD outbreak. During the past 11 years 47% of dairy herds examined in Saudi Arabia have experienced FMD initially as sub-clinical disease. For the remaining 53%, waning vaccinal protection did not suppress clinical disease in the initially infected animals, and these showed severe rather than mild signs. Hence, in such herds there was a very low initial level of subclinical infection, so PoV was more effective, and the timing of PoV was found to give a good correlation with cumulative herd incidence: an early PoV resulted in low prevalence of clinically infected animals whilst late PoV permitted high prevalence. PoV timing can thereby be used in tandem with FDI as a predictive tool for future outbreaks, estimating the final cumulative incidence (or prevalence) of clinical FMD cases.