Dysplasia epiphysealis hemimelica, which is also known as Trevor disease, is a rare developmental lesion that is histologically identical to an osteochondroma. The more common solitary osteochondroma and multiple hereditary osteochondromatosis typically are located on the metaphysis of long bones, occasionally on the diaphysis of long bones, and on flat bones, but never on the epiphysis. The index lesion is intra-articular and characteristically involves only half of the joint (hemimelic). The osseous portions of the lesion distinguish it from synovial chondromatosis, which always consists of multiple lesions usually dispersed throughout the joint. The osteochondroma of dysplasia epiphysealis hemimelica, in addition to being hemimelic, is initially a solitary lesion, although portions may break off, producing multiple pieces17. It is typically found in the joints of the lower extremity, with a predisposition for the medial femoral condyle, the distal aspect of the tibia, and the talus1,2,4,5,7,12,15,21,25. Mouchet and Belot16, in 1926, were the first to report this entity, and they called it tarsomegalie. Trevor25 described ten patients in 1950 and used the term tarso-epiphysial aclasis. In 1956, Fairbank5 reported on fourteen patients and renamed the condition dysplasia epiphysialis hemimelica. We report the cases of two patients who had a subluxation of the hip that was found to be associated with an isolated intra-articular osteochondroma of the acetabulum. These two cases are presented because of the isolated and unique acetabular location of the lesion. CASE 1. A five-year-old girl was referred to one of the authors (H. A. P.) because of a several-month history of bilateral discomfort of the lower limb. The patient's father had noted a change in the child's gait a few days before she was seen …