How to assess a patient's functional physical mobility in non-laboratory situations has been a problem for the medical services for a long time. A measurement system using accelerometry (DynaPort ADL Monitor) has been developed to cope with this problem. It is a lightweight recorder that is worn around the waist. It uses accelerometry to ambulatory monitor the Activities of Daily Living (ADL). In this study the reliability of the monitor in two working situations of one patient has been assessed: maintenance mechanic and messenger in a city hall. All actions were simultaneously videotaped. The video recording was interpreted by an observer. Independent of this, the acceleration signals were translated by the monitor software into one of the basic postures or movements: locomotion, standing, sitting, lying and playing. Every second the monitor and observer interpretation were compared and maximal and minimal validity percentages were calculated for all classes and for the total measurement. The overall (minimal) validity was found to be higher than 86%. The maximal validity was 93% of the total measurement in the patient's job as a maintenance mechanic. Considering the simple instrumentation of the ADL monitor this validity is considered good.