Previous studies have linked essential sustained hypertension with Chlamydia pneumoniae (C. pneumoniae) and changes in intima-media thickness (IMT) of carotid arteries. The aim of this study was to examine if similar associations exist in subjects with white-coat hypertension. C. pneumoniae IgA and IgG antibody titers were measured in 125 patients with white-coat hypertension and 54 normotensives. All participants underwent a 24 h ambulatory blood pressure (BP) monitoring, clinic BP readings and common-internal carotid artery IMT measurements. Seventy subjects of the white-coat group (56%) and 15 of the control group (27.8%) had IgG titers >/=80 (crosstabs; P<0.001). IgA titers were elevated in 75 subjects of the white-coat group (60%) and 10 (18.5%) of the control group (crosstabs; P<0.001). The IMT of the carotid arteries in the white-coat group was significantly higher than that of the normotensive group (t-test; P<0.001 and P<0.001, respectively). In contrast, carotid IMT did not differ between C. pneumoniae-seropositive and C. pneumoniae-seronegative groups concerning both IgG and IgA antibody titers. Our findings suggest that both C. pneumoniae antibody titers and carotid IMT were increased in subjects with white-coat hypertension. The preceding associations strengthen prior evidence in favor of the opinion that white-coat hypertension is not an innocent phenomenon.