Heterotopic bone formation (HBF) is one of the many causes of painful limb swelling and is well recognized as posing clinical, radiological, and pathological diagnostic difficulties. We have evaluated the use of sonography to determine its role in reaching a diagnosis at a time when symptoms are maximal and the radiological diagnostic features absent. Serial ultrasound examination identified unique 'zone phenomea' in a heterogeneous group of eight patients before the classic radiological features appeared. The zone transformation matches the evolution and maturation process described pathologically and forms the basis of an early definitive diagnosis in all cases. The sonographic features are specific for the condition, unlike the scintigraphic appearances. Although CT confirms the peripheral location of the calcification, the low cost, safety, and general availability of ultrasound scanning make it the optimum imaging method for diagnosing and monitoring heterotopic bone formation.