BACKGROUND: The purposes of this study were (1) to determine how supine versus upright patient position affects the measurement of diaphyseal clavicle fracture displacement, (2) to describe the incidence of progressive displacement in the peri-injury period, and (3) to investigate variables associated with the progressive displacement. METHODS: Between 2013 and 2015, patients aged 14 years or older presenting with a diaphyseal clavicle fracture within 7 days of injury were included (N = 50). A well-defined radiographic protocol was established. Nine patients underwent surgery after the second follow-up, and the remaining 41 patients, who did not undergo surgery, received the full complement of measures at the first, second, and third follow-up time points. The second follow-up (8-21 days after injury) and third follow-up (22-60 days after injury) had the same defined radiographic protocol as the first visit. The amount of displacement and angulation was measured in both the supine and upright positions on the initial injury radiographs and subsequent follow-up radiographs. RESULTS: Vertical translation was 2.4 mm (95% confidence interval, 1.8-3.0 mm) greater and angulation was 3.9 degrees (95% confidence interval, 3.3 degrees -4.6 degrees ) greater in the upright position. Progressive displacement occurred in 16 patients (32%). Older age (P = .015) and ipsilateral shoulder girdle or chest wall injury (P = .007) were significantly associated with progressive displacement. CONCLUSIONS: Upright radiographs evaluate maximal displacement in diaphyseal clavicle fractures. Close follow-up of nonoperatively treated clavicle fractures is warranted. Progressive displacement was more likely in older patients and/or those who had ipsilateral shoulder girdle or chest wall injury.