Background: The aetiology of postconcussion syndrome (PCS) following mild traumatic brain injury (mTBI) remains controversial. Identifying acute PCS (within the first 14 days after injury) may optimise initial recovery and rehabilitation, identify those at risk and increase understanding of PCS. Objective: To examine predictors of acute outcome by investigating the relationship between preinjury psychiatric disorder, demographic factors, injury related characteristics, neuropsychological and psychological variables and acute PCS. Methods: Prospective study of consecutive trauma admissions to a level 1 trauma hospital. The final sample comprised 90 patients with mTBI and 85 non-brain injured trauma controls. Individuals were administered a PCS checklist, and neuropsychological and psychological measures. Multiple imputation of missing data in multi-variable logistic regression and bivariate logistic regressions were used to predict acute PCS at a mean of 4.90 days after injury. Results: Diagnosis of acute PCS was not specific to mTBI (mTBI 43.3%; controls 43.5%). Pain was associated with acute PCS in mTBI. The strongest effect for acute PCS was a previous affective or anxiety disorder (OR 5.76, 95% CI 2.19 to 15.0). Females were 3.33 times more likely than males to have acute PCS (95% CI 1.20 to 9.21). The effect of acute post-traumatic stress and neuropsychological function on acute PCS was relatively small. Higher IQ was associated with acute PCS. Conclusions: There is a high rate of acute PCS in both mTBI and non-brain injured trauma patients. PCS was not found to be specific to mTBI. The use of the term PCS may be misleading as it incorrectly suggests that the basis of PCS is a brain injury.