In most cases ankle sprains involve external compartment ligaments. In particular, the anterior talofibular ligament is involved alone in 70% of cases and together with calcaneofibular and posterior talofibular ligaments in the remaining 30% of cases. To investigate the potentials and the possible limitations of high-frequency US (7.5-13 MHz) for the preliminary assessment of the extent of damage of the capsulo-ligamentous lesions of the external ankle compartment, 25 athletes with clinical diagnosis of sprain trauma were examined with conventional radiology. Morphology and structure were studied from the semiologic point of view. US findings were compared with MR results. In all patients, US showed anterior talofibular ligament lesions alone in 13 patients and associated with calcaneofibular ligament lesions in 12 patients. The posterior talofibular and the interosseous talocalcaneal ligaments were never demonstrated by US. In 4/13 patients diagnosed by US as having isolated anterior talofibular lesions, MRI demonstrated a coexisting calcaneofibular lesion (4/13) and, in one of them, a posterior talofibular and interosseous talocalcaneal ligaments lesion. The comparison of US and MR findings in the patients US had diagnosed as having associated anterior talofibular and calcaneofibular ligaments lesions, showed 100% agreement; MRI allowed the demonstration of the lesions in the posterior talofibular and interosseous talocalcaneal ligaments in 2/12 patients. Considering the statistical prevalence of the anterior peroneal-astragalic lesions caused by ankle sprains, the use of high-frequency US as the first diagnostic approach seems justified. Nevertheless, MRI is a fundamental complement for accurately assessing damage extent.