The aim of this study was to examine the levels of circulating leukocytes and lymphocyte subpopulations before and immediately after experimentally induced swine dysentery. Twenty-one healthy crossbred pigs (approximately 22 kg) were orally inoculated with Brachyspira hyodysenteriae. Blood was sampled before inoculation and when clinical signs of swine dysentery occurred. Pigs that remained healthy were sampled when killed. Total and differential white blood cell counts were performed, and lymphocyte subpopulations were analysed using flow cytometry. Following a mean incubation period of 13 days, 12 pigs developed swine dysentery, whereas nine remained healthy throughout the study. Before inoculation, pigs that subsequently developed swine dysentery displayed higher levels of circulating gamma delta T cells (mean +/- se; 30.7 +/- 3.5 %) compared with pigs that remained healthy (14.9 +/- 1.4 %). Sick animals also displayed lower levels of CD8 cells (24.6 +/- 1.5 %), cytotoxic/suppressor T cells (10.9 +/- 1.3 %) and CD4 CD8 T cells (8.1 +/- 1.0 %) than the pigs that remained healthy (34.9 +/- 3.1 %; 17.6 +/- 2.0 %; 13.6 +/- 2.3 %). No difference was observed in leukocyte counts before inoculation. At onset of swine dysentery, there was an increase in monocytes (from 1.5 +/- 0.2 x 10 to 3.8 +/- 0.5 x 10 l) and CD4 CD8 T cells (from 5.8 +/- 0.9 to 8.9 +/- 0.7 %). In conclusion, gamma delta T cells and CD8 cells may be associated with susceptibility to experimentally induced swine dysentery, whereas monocytes and CD4 CD8 T cells appear to be the major responding leukocytes during the disease.