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Several methods are commonly used to predict lower-limb-length discrepancy (LLD) on pelvic radiographs. It is not clear how the lower-limb length of patients with unilateral developmental dislocation of the hip (DDH) changes and whether a pelvic radiograph is reliable to predict LLD. In this study, we analyzed the characteristics of LLD in patients with unilateral DDH by measuring full-length standing anteroposterior radiographs.The radiographic data of all patients with unilateral DDH who met the inclusion criteria from March 2011 to May 2016 were retrospectively reviewed. These data included femoral length, tibial length, skeletal limb length, and distance from the lesser trochanter to the tibial plafond. We also compared LLD between patients with Hartofilakidis type-II DDH and those with type III.Sixty-seven patients (12 male and 55 female) were included. The tibial length, skeletal limb length, and lesser trochanter-tibial plafond distance were significantly greater (p < 0.001, p = 0.040, and p < 0.001, respectively) on the ipsilateral (DDH) side, compared with the contralateral side, in 51 patients (76%), 43 patients (64%), and 52 patients (78%), respectively, with the values on the ipsilateral side exceeding those on the contralateral side by an average of 4.6 mm (range, 0.4 to 17.5 mm), 7.0 mm (range, 0.3 to 21.1 mm), and 10.0 mm (range, 1.1 to 28.8 mm), respectively. The femoral length did not differ significantly between the 2 sides (p = 0.562). There was also no significant difference in LLD, femoral length, tibial length, skeletal limb length, or lesser trochanter-tibial plafond distance between patients with Hartofilakidis type II and those with type III (p > 0.05).Patients with unilateral DDH, regardless of whether the hip dislocation is low or high, may present with LLD derived from both the femur and the tibia. This LLD includes a greater ipsilateral tibial length, skeletal limb length, and lesser trochanter-tibial plafond distance in most patients and an unpredictable femoral length. Using the lesser trochanter on pelvic radiographs to predict LLD is not reliable. The use of full-length standing anteroposterior radiographs for preoperative templating is advisable for this special group of patients.Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.