Sever's disease (calcaneal apophysitis) is a self-limiting condition seen in physically active children. Although there is controversy about the radiographic appearance, some reports propose the importance of fragmentation of the secondary nucleus in the diagnosis of Sever's disease. We studied secondary nucleus of the calcaneus with ultrasonography. Twenty-one symptomatic heels of 14 children were examined. All these heels showed fragmentation of the secondary nucleus on both conventional radiograph and sonography. Ultrasonographic examination also showed 2 retrocalcaneal bursitis. Our initial data showed that sonography may be valuable in the diagnosis of Sever's disease.