The incidence of primary bone tumor of the acetabulum is 1-2% of all the bone tumors. Degenerative cystic change can occur in patients with hip dysplasia and this can be misdiagnosed as primary bone tumor. We have experienced 2 patients with painful dysplastic hips that were treated with curettage and autogenous bone graft for a cystic lesion that was originally misdiagnosed as primary bone tumor. The patients'hip pain and limitation of motion recurred, and so Bernese periacetabular osteotomy was performed for the dysplastic hips. At over one year follow-up, the corrected hips showed a painless and good range of motion.