Background Rosacea is a chronic inflammatory condition that affects the skin of the face and the eyes. Erythematotelangiectatic rosacea is characterized by flushing, oedema and telangiectasia. Patients with rosacea demonstrate elevated densities of Demodex mites in their skin compared with controls. A bacterium (Bacillus oleronius) isolated from Demodex mites from a patient with papulopustular rosacea has been demonstrated to produce antigenic proteins that may play a role in papulopustular and ocular rosacea. Objectives To establish whether there was a correlation between the reactivity of sera from patients with erythematotelangiectatic rosacea to Bacillus antigens, and to characterize the proteins to which these patients showed reactivity. Methods Serum samples from patients with erythematotelangiectatic rosacea and controls were examined for reactivity to Bacillus proteins by Western blot analysis. Proteins to which the sera reacted were excised from gels, trypsin digested, and putative identities were assigned following liquid chromatography-mass spectrometry (LC-MS) analysis. Results Eighty per cent (21 ⁄26) of patients with erythematotelangiectatic rosacea showed serum reactivity to the 62- and 83-kDa proteins of B. oleronius, compared with 40% (9 ⁄22) of controls (P = 0Æ004). The 62-kDa protein was characterized by LC-MS and showed homology to groEL chaperonin, which provokes a strong immune response in mammals. The 83-kDa protein showed homology to aconitate hydratase, of which expression is increased in bacteria under oxidative stress, and which is highly immunogenic. Conclusions The majority of patients with erythematotelangiectatic rosacea show serum reactivity to two proteins from B. oleronius, suggesting that this bacterium may play a role in the induction of this condition. The two proteins to which patient sera reacted were found to be similar to a heat shock protein and an enzyme involved in regulating the stress response of the bacterium.