Measurement of actual bionator wear has so far been confined to a few studies involving only small samples. In this study a micro-electronic timing device was used in a greater number of patients (53 children) after the system proved to be reliable in a pretest involving 18 subjects. The patients were 9-14 years of age and had a timing device fitted in their bionators over a period of 3-6 months. The data were read from the memory chip at the regular treatment sessions every 3-4 weeks. There was evidence that the patients averaged 7.65 hours per day of wear, thus fulfilling only 50-60 per cent of the orthodontist's requirements. Further details and implications of the measurements are discussed.