Paratuberculosis was first reported in sheep in Australia in the early 1980s. In the subsequent 20 years the number of infected flocks in the country has risen to over 500. While early reports suggested that the biological and economic impact of the disease might be small, in the late 1990s an increasing number of sheep farmers reported high mortality rates in their flocks which they attributed to paratuberculosis. In one of these flocks, which contained about 10,000 Merino sheep, we commenced a study in April 2000 with the objectives of estimating the paratuberculosis-attributable death rate and relating the death rate to the seroprevalence of paratuberculosis and the rate of faecal shedding of MAP. Paratuberculosis vaccination of the whole flock occurred towards the end of the first year of the study. Estimates of the paratuberculosis-attributable mortality rate prior to vaccination were made by performing necropsies of all sheep which died on 20 days of each year. Estimates of the crude mortality rate were made by deduction following annual counts. The paratuberculosis-attributable mortality rate in the adult sheep (12 months or greater)in the first year was 14.6%. We present also data for seroprevalence, faecal excretion rate, pathology and mortality rates for both years of the study.