The purpose of this study was to assess the accuracy of an ambulatory monitor (Takeda 2421; A&D company) which measures blood pressure using oscillometric and Korotkoff methods, in children. Blood pressure was measured simultaneously by the device and two observers in 529 children aged 7-15 years. The Korotkoff method received BHS grade A for sitting and standing readings in all age groups except for age 13-15 years (grade C). The diastolic readings (phase V) received grade B for sitting and C when standing. The oscillometric method received grade C or worse for systolic and diastolic blood pressure in both sitting and standing positions except for systolic readings in those aged 13-15 years (grade B). During the ambulatory phase 30% of the Korotkoff readings and 4% of the oscillometric readings gave an error code. The Takeda 2421 ambulatory monitor achieved generally satisfactory grades using the Korotkoff system but the oscillometric readings were generally unacceptable. Ambulatory devices should be specifically validated in children before being accepted into clinical paediatric practice.