The effect of a muscle-bone interface on the temperature distribution during scanned, focused ultrasound hyperthermia was studied in vivo in dogs' thighs. The detailed temperature maps showed a sharp temperature maximum at the interface. The magnitude of the temperature elevation was between two and four times as large as obtained in resting muscle with similar intensities. The muscle tissue close to the bone could be heated to a therapeutic temperature level without overheating the bone when higher frequency or multiple-focused beams were used. The results showed that bone pain associated with ultrasound hyperthermia treatments is probably caused by extreme temperatures in bone, which can be avoided by sonicating the tumour in such a manner that the intensities at the bone surface are low enough so as not to cause large temperature elevations. Therefore, treatment planning which takes into account the location of bones is necessary for successful, clinical utilization of scanned, focused ultrasound hyperthermia.