A retrospective/prospective radiographic study documented 55 cases of moderate to severe hip dysplasia, with varying degrees of shallowing of the acetabulum, flattening or loss of the femoral head, widening or loss of the femoral neck, and femoral diaphyseal abnormalities in northern koalas (Phascolarctos cinereus) in the San Diego Zoo (San Diego, California, USA) collection. For the retrospective study, historic radiographs were examined when availble. For the prospective study, three standard views (ventrodorsal extended leg, ventrodorsal frog leg, and lateral extended leg) were used. A scoring system was developed using four areas (acetabulum, femoral head, femoral neck, and femur) and ranges of 0 to 5 (0 = not affected to 5 = severely affected) for each area, creating a total score out of 40. Scores were graded as follows: 0-2 = normal dysplasia; 3-6 = mild dysplasia; 10-19 = moderate dysplasia; and 20-40 = severe dysplasia. Thirty koalas were graded as severe, 25 koalas as moderate, and 38 koalas as excellent or mild. Affected koalas may or may not demonstrate gait abnormalities. Mild to severe degenerative joint disease may develop and symptoms may be alleviated with glucosamine/chondroitin sulfate and nonsteroidal anti-inflammatory drugs. The etiology of hip dysplasia in koalas is not currently understood.