Proportions and absolute numbers of circulating leukocyte populations and lymphocyte subpopulations (T- and B-cells) were determined in 33 patients with Crohn's disease (group CD), and were compared with those of age- and sex-matched healthy subjects. Group CD comprised 15 patients with newly diagnosed, short-standing, and untreated CD (group CD 1) and 18 patients with long-standing, previously drug treated CD (group CD 2). All CD groups showed a significant absoulte leukocytosis, based on a significant absolute and relative increase of the neutrophils, and, as far as group CD and CD 1 were concerned, also of the absolute number of monocytes. In group CD 1, absolute lymphocyte and relative as well as absolute T-cell numbers were close to normal. In contrast, in group CD 2 absolute as well as relative numbers of lymphocytes and absolute numbers of T-cells were highly significantly reduced, whereas the reduction of the relative T-cell concentration barely reached significance. The proportion of B-cells was significantly above normal in all patient groups, the absolute number in group CD only. Also group CD 1 showed considerably, though statistically insignificantly, higher than normal absolute B-cell numbers. In group CD 1 , there was an inverse correlation between absolute T-cell numbers and disease activity, and between absolute lymphocyte numbers and duration of disease. These data indicate that there is no gross numerical reduction of the carriers of the cell-mediated immunity as a primary predisposing factor for CD, but that a reduction of these cells occurs in the circulation after the disease has started.