Oscillometric devices are being widely used for ambulatory, home and office blood pressure (BP) measurement. However, even successfully validated oscillometric devices fail to provide accurate measurements in some patients. This study investigated the prevalence, the reproducibility and the characteristics of the phenomenon of unreliable oscillometric BP (UOBP) measurement. A total of 5070 BP measurements were obtained simultaneously (Y connector) using a professional oscillometric device (BpTRU) and a mercury sphygmomanometer in 755 patients (1706 visits). UOBP readings were defined as those with >10 mm Hg difference (systolic or diastolic) between the two methods. UOBP was found in 15% of systolic and 6.4% of diastolic BP measurements. In all, 18% of the participants had UOBP in their first but not their second visit, or the reverse. However, 49% of these participants had at least one more UOBP visit after their second visit within the study database. Patients with persistent UOBP were more likely to be female and had lower arm circumference. The systolic BP discrepancy between the two methods was associated with pulse pressure (r=0.41) and inversely with diastolic BP (r=0.40) and arm circumference (r=0.30), whereas the diastolic discrepancy with diastolic BP (r=0.61) and inversely with pulse pressure (r=0.32). There was a consistent significant trend for larger systolic BP discrepancy and smaller diastolic from the lower to the higher pulse pressure quintile (P<0.0001). A decreasing arm circumference was a significant predictor of persistent UOBP. These data suggest that the UOBP measurement is particularly common, not very reproducible and mainly affected by pulse pressure and arm circumference.