One of the difficulties in clinical assessment is how to obtain accurate data in the "real world". This paper describes the Dynaport ADL Monitor, an accelerometry-based system for ambulatory monitoring of the activities of daily living (ADL). In previous studies the monitor has proved a success with adults. To validate the system for use with children, 9 h of various activities conducted by nine children were measured and videotaped at the same time. All postures and activities were divided into one of five main categories: standing, sitting, lying, locomotion and swing/seesaw. The video pictures were evaluated by an observer. Independent of this, the acceleration signals were translated by the DynaPort ADL monitor software, and were compared to the video pictures. Minimal and maximal validity percentages were calculated for each of the main classes, for each individual measurement and overall. To estimate monitoring performance the 10 sets of measurements are regarded as representative samples of children's daily activity. The overall minimal and maximal validity are 73.15 +/- 1.96 x 4.48 and 91.31 +/- 1.96 x 1.75 weighed standard deviation.