In an almost full-grown male of 17 years, functional and morphological changes were followed during Herbst treatment and subsequent control. Retrognathism of the mandible was overcorrected, and then normalized by reverse headgear to the maxilla. Partial relapse due to insufficient cooperation during retention necessitated retreatment. Treatment resulted in marked dentoalveolar changes, increase in mandibular length and ramus height, apposition and remodelling of condylar heads and flattening of the temporal joint surface. After treatment masseter strength was 40 per cent reduced, probably due to prolonged hypofunction.