Besides clinical indices, acute phase reactants and measurement of permeability of the gut immunological parameters have been proposed for assessment of clinical activity in Crohn's disease (CD). The latter refers in particular to the number of activated peripheral T cells (APT) which are found to be increased in patients with CD and ulcerative colitis. Further analysis of the subset of APT revealed that in CD their number is correlated to the histopathological ratings and the number of activated T cells of the affected mucosa. A major subset of APTs in CD and ulcerative colitis expresses receptors for IgA (Fc-alpha-R). This T cell subset seems to be characteristic of patients with inflammatory bowel diseases, exhibiting a specificity of 88% and a sensitivity of 92% for CD as compared with non-inflammatory bowel diseases. Methodological complexity turned out to be the major disadvantage of assessment of APT and their subsets.