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Nonunion is a rare complication of a fracture of the clavicle, but its occurrence can compromise shoulder function. The aim of this study was to evaluate the prevalence of and risk factors for nonunion in a cohort of patients who were treated nonoperatively after a clavicular fracture.Over a fifty-one-month period, we performed a prospective, observational cohort study of a consecutive series of 868 patients (638 men and 230 women with a median age of 29.5 years; interquartile range, 19.25 to 46.75 years) with a radiographically confirmed fracture of the clavicle, which was treated nonoperatively. Eight patients were excluded from the study, as they received immediate surgery. Patients were evaluated clinically and radiographically at six, twelve, and twenty-four weeks after the injury. There were 581 fractures in the diaphysis, 263 fractures in the lateral fifth of the clavicle, and twenty-four fractures in the medial fifth.On survivorship analysis, the overall prevalence of nonunion at twenty-four weeks after the fracture was 6.2%, with 8.3% of the medial end fractures, 4.5% of the diaphyseal fractures, and 11.5% of the lateral end fractures remaining ununited. Following a diaphyseal fracture, the risk of nonunion was significantly increased by advancing age, female gender, displacement of the fracture, and the presence of comminution (p < 0.05 for all). On multivariate analysis, all of these factors remained independently predictive of nonunion, and, in the final model, the risk of nonunion was increased by lack of cortical apposition (relative risk = 0.43; 95% confidence interval = 0.34 to 0.54), female gender (relative risk = 0.70; 95% confidence interval = 0.55 to 0.89), the presence of comminution (relative risk = 0.69; 95% confidence interval = 0.52 to 0.91), and advancing age (relative risk = 0.99; 95% confidence interval = 0.99 to 1.00). Following a lateral end fracture, the risk of nonunion was significantly increased only by advancing age and displacement of the fracture (p < 0.05 for both). On multivariate analysis, both of these factors remained independently predictive of nonunion (p < 0.05), and, in the final model, the risk of nonunion was increased by a lack of cortical apposition (relative risk = 0.38; 95% confidence interval = 0.25 to 0.57) and advancing age (relative risk = 0.98; 95% confidence interval = 0.97 to 0.99).Nonunion at twenty-four weeks after a clavicular fracture is an uncommon occurrence, although the prevalence is higher than previously reported. There are subgroups of individuals who appear to be predisposed to the development of this complication, either from intrinsic factors, such as age or gender, or from the type of injury sustained. The predictive models that we developed may be used clinically to counsel patients about the risk for the development of this complication immediately after the injury.Prognostic study, Level I-1 (prospective study). See Instructions to Authors for a complete description of levels of evidence.