Background: Radiographic measures such as the rib vertebral angle difference (RVAD), Cobb angle, and space available for the lung (SAL) help to guide treatment and measure treatment effects in patients with infantile idiopathic scoliosis. This study aimed to evaluate the intraobserver and interobserver reliability of these radiographic measures. Methods: Forty-five spine radiographs of skeletally immature patients (age, two months to four years) with infantile idiopathic scoliosis were measured with use of Surgimap software. Three pediatric orthopaedic surgeons and a pediatric orthopaedic fellow identified the major curve apex, rib-vertebra phase, Cobb angle, and end vertebrae and calculated the RVAD and SAL values at two separate time points. Interobserver and intraobserver reliability of the RVAD, Cobb angle, and SAL values were assessed with use of intraclass correlation coefficients (ICCs). Fleiss kappa coefficients were calculated for categorical variables. Results: The RVAD (ICC = 0.86 to 0.92) and Cobb angle (ICC = 0.99) measurements had high reliability. The SAL value had substantial interobserver reliability (ICC = 0.66) and moderate intraobserver reliability (ICC = 0.73). Despite the high agreement for the Cobb angle, the choice of the major curve vertebrae (kappa = 0.19 to 0.39) and apical vertebra varied (kappa = 0.57 to 0.62). Observers were more likely to choose the same apical vertebra in large curves (r = 0.483, p = 0.001). The agreement for the apical rib-vertebra phase was substantial (kappa = 0.67). Paired RVAD measurements fell within ≤10° of each other in 82% of cases, but the remaining 18% of the RVAD measurements showed >10° of variation. Conclusions: Measurements used to guide treatment of infantile idiopathic scoliosis curves were reliable despite standard radiographic measurement error and the difficulty in obtaining quality images in young patients. Clinicians are dependent on seemingly objective radiographic data. The reliability of the Cobb angle and RVAD measurements in infantile scoliosis was high but not devoid of variability that could skew the ability to accurately and reliably suggest the best course of treatment. The SAL value was a less reliable measure. Treatment recommendations for infantile idiopathic scoliosis should rely on the synthesis of objective and clinically subjective data, as variations in radiographic measurements can lead to inconsistencies in management and to inconsistent treatment outcomes.