This study is the first conducted in Spain to evaluate antimicrobial susceptibility of field isolates of Brachyspira hyodysenteriae. One hundred and eight isolates of the bacterium, recovered from different Spanish swine farms between 2000 and 2007, were investigated. The minimum inhibitory concentrations (MIC) of erythromycin, tylosin, tiamulin, valnemulin, clindamycin and lincomycin were determined using a broth microdilution technique. Most of the isolates showed poor susceptibility to erythromycin (MIC(90)>256 microg/ml), tylosin (MIC(90)>256 microg/ml), clindamycin (MIC(90)>4 microg/ml) and lincomycin (MIC(90)=128 microg/ml). Reduced susceptibility to tiamulin and valnemulin was observed with a MIC>2 microg/ml in 17.6% and 7.41% of the B. hyodysenteriae isolates, respectively. Moreover, a survival analysis permitted the detection of an increasing trend in the MIC values for almost all the antimicrobials used in the treatment of swine dysentery when comparing recent isolates (from 2006 to 2007) with those recovered in earlier years (between 2000 and 2004).