We describe the first steps in the development of a wearable measurement device for measuring a subject's three-dimensional acceleration. The ultimate aim is a standard measurement instrument integrated in a belt buckle that allows objective evaluation of treatment and rehabilitation measures in patients, in particular for disabling chronic diseases such as multiple sclerosis. In a first step we combined standard hardware elements to record test data from healthy volunteers. We then developed algorithms to automatically distinguish between different stages of activity, such as jogging, walking, lying, standing and sitting, and to detect and count steps. Distinction between standing and sitting is the most difficult to accomplish. As a first validation, we calculated the distance traveled from data of 17 experiments and a total of 4.5 h, for which one proband was walking and running for a known distance, and compared the results with two commercially available pedometers. We could show that the relative error for the ActiBelt is only half of that for the two pedometers. Apart from developing much smaller, robust and integrated hardware, we describe ideas on how to develop algorithms that allow extraction of a "baseline step pattern" in analogy to baseline ECG to define and detect clinically relevant deviations.