Background: Acetabular dysplasia is recognized as a cause of early degenerative hip osteoarthritis. The purpose of this study was to prospectively determine the early clinical presentation of symptomatic acetabular dysplasia in skeletally mature patients. Methods: Fifty-seven consecutive skeletally mature patients with a total of sixty-five symptomatic hips were diagnosed with symptomatic acetabular dysplasia on the basis of the history, physical examination, and radiographs. These fifty-seven patients were enrolled in this study and were followed prospectively for a minimum of twenty-four months postoperatively. Results: The study group included forty-one female patients (72%) and sixteen male patients (28%) with a mean age of twenty-four years. All were treated with a periacetabular osteotomy and were followed for a minimum of twenty-four months. The initial presentation was insidious in 97% of the hips, and the majority (77%) of the hips were associated with moderate-to-severe pain on a daily basis. Pain was most commonly localized to the groin (72%) and/or the lateral aspect of the hip (66%). Activity-related hip pain was common (88%), and activity restriction frequently diminished hip pain (in 75% of the cases). On examination, thirty-one hips (48%) were associated with a limp; twenty-five (38%), with a positive Trendelenburg sign; and sixty-three (97%), with a positive impingement sign. The mean time from the onset of symptoms to the diagnosis of hip dysplasia was 61.5 months. The mean number of health-care providers seen prior to the definitive diagnosis was 3.3. The mean Harris hip score improved from 66.4 points preoperatively to 91.7 points at a mean of 29.2 months after the periacetabular osteotomy. Conclusions: The diagnosis of symptomatic acetabular dysplasia is commonly delayed, and procedures other than a pelvic reconstructive osteotomy are frequently recommended. The diagnosis of developmental dysplasia of the hip should be suspected and investigated when a skeletally mature, young, active patient has a predominant complaint of insidious activity-related groin pain and/or lateral hip pain. Level of Evidence: Prognostic Level IV. See Instructions to Authors for a complete description of levels of evidence.