In sheep clinically affected with paratuberculosis, two distinct forms of microscopical pathology were recognized, related to a high or a low degree of mycobacterial colonization ("multibacillary" or "paucibacillary" presence). These forms were characterized by different types of cellular infiltrate in the ileal mucosa and submucosa. Statistical analysis demonstrated strong correlations between the presence of large numbers of acid-fast organisms and macrophage infiltration, and between small numbers and lymphocyte infiltration. Correlations also existed between high numbers of acid-fast bacteria and a positive serum antibody test results; and between the presence of giant cells and lymphocytes in the gut. This study suggests that in ovine paratuberculosis the same clinical and gross pathological changes can result from different pathogenetic mechanisms.