The present study investigates the role of subclinical personality traits as determinants of leader development over time. In previous literature subclinical traits have been identified as potential causes of leader derailment. However leader development researchers have argued that developmental interventions based on increasing self-awareness may be effective at mitigating the negative effects of these character flaws. Using a multi-wave multi-method longitudinal study of military school cadets we evaluate the impact of subclinical traits on externally-rated measures of leader development over a three year period. Results demonstrated that adding subclinical traits to models of development significantly increased model fit and that the relationship between training is more complicated than originally thought. Moreover subclinical traits were associated with different developmental trajectories over time. Implications for leadership development research and practice are discussed.