A case study of the economic impact of Johne's disease in an Irish dairy herd is described. An epidemiological investigation concluded that the purchase of 20 heifers from the Netherlands in 1993 introduced Johne's disease to the herd. The practice of feeding pooled colostrum/milk was considered to have disseminated Mycobacterium avium subspecies paratuberculosis (MAP) widely within the herd. Farm performance between 1993 and 2003 declined substantially, as a result of reduced milk yields, increased culling and reduced cull cow values. This in turn reduced the profit margin per litre milk sold and per cow. The performance relative to peers also deteriorated over the study period, where the farm performance was superior to that of peers until the late 1990's, but was markedly worse than that of peers by 2002.