A purported hallmark of temperament characteristics is that they appear very early in the course of development and are persistent across time and situation. There is, however, a small, but growing cadre of research findings that question this traditional view. It may be that temperament characteristics are not necessarily established during the first few months of development or are not stable over time. A secondary analysis of the New York Longitudinal Study (NYLS) temperament data was conducted to investigate the hypothesis that temperament has a complex developmental course over the first five years of development. A latent growth curve analysis of difficult temperament suggests that temperament development follows a non-linear trajectory. This finding, in concert with related findings, suggests the need for a broader discussion of the possible developmental processes that underlie these patterns. Copyright © 2007 John Wiley & Sons, Ltd.