Background Supracondylar humerus fractures (SCH) are common upper extremity fractures in children and are usually treated by closed reduction and percutaneous pinning. Post-operative management may cause complications, but the difference between cast and splint has not been closely investigated. Purpose Our objective was to compare casting and splinting of SCH fractures with respect to post-operative complications. Patients and methods We reviewed 1,146 pediatric SCH fractures that were reduced, percutaneously pinned, and immobilized by cast or splint. Open fractures, openly reduced fractures, and pre-operative neurological injuries were excluded. Over the course of immobilization, we noted if the initial cast or splint was maintained and if the patient returned due to complications. Results Post-operative casting was performed on 1,091 (95.2%) fractures and 55 (4.8%) were splinted. Age was a significant factor, increasing the likelihood of splinting by 12% with each year of age (p = 0.023). A total of 28 patients (2.4%) returned for unscheduled visits due to immobilization complaints, infection, and pain, but the rate difference between cast and splint was negligible. Reoperation was required for five patients (0.4%), and more likely for splinted fractures (p = 0.021). After controlling for age, splinting was still associated with reoperation (OR: 15.1, p = 0.004). Conclusions Although complications inevitably exist, both casting and splinting are effective immobilization methods. Both resulted in few complications such as post-operative discomfort, pain, infection, loss of reduction, or damage. It was difficult to evaluate significance with few splinted cases, but considering no major differences between splinted and casted fractures, clinicians should consider splinting to reduce the cost associated with casting.