The objective of the study is to investigate ambulatory blood pressure monitoring (ABPM) in a sample of Spanish nonagenarians. We also analyzed the misdiagnosis of hypertension and investigated blood pressure (BP) control in treated hypertensive nonagenarians. Twenty-four-hour ABPM was undertaken in a group of 42 nonagenarians. The 24-h mean, daytime BP, nighttime BP and heart rate (HR) were extracted from the ABPM. Sociodemographic data, the ability to perform basic daily activities, measured by the Barthel index (BI) or instrumental activities revealed by the Lawton and Brody index (LI), cognition, and comorbidity were evaluated. Thirty-one subjects were receiving antihypertensive drug treatment. Twenty-four hour, daytime and sleeping pressures averaged 130/65, 131/68 and 128/63mmHg, respectively. Seventeen (40.5%) of the 42 patients had a daytime BP of 135/85 or higher. In terms of the BP pattern, 8 (19%) subjects were dippers, 19 (45%) non-dippers, and 15 (36%) were risers. Five (45.46%) out of 11 patients with no evidence of hypertension (normotensive patients) had a daytime BP of 135/85 or higher. The mean daytime BP was 135/85 or higher in 12 (38.7%) out of 31 nonagenarians who had previously received therapy for hypertension. In, conclusion a high prevalence of hypertension, misdiagnosis and inadequate BP control was found in nonagenarians treated for hypertension.