This study examined the stability of temperament characteristics in children over a 6-year period as well as the reliability of Rothbart's infant and child temperament measures. Thirty-seven mothers rated their firstborns' temperaments using the Infant Behavior Questionnaire (IBQ) at 3 months and the Children's Behavior Questionnaire (CBQ) at 6 years of age. Paired t tests between matched IBQ/CBQ subscales revealed that children were rated significantly higher in all behavioral subscales at 6 years, with the exception of Soothability, which remained stable. Correlations among matched IBQ/CBQ subscales showed significant positive relationships between Soothability and Smiling over the 6-year period. Estimates of internal consistency on three of the six IBQ subscales were lower than Rothbart's reported values, whereas the CBQ values were similar. Findings suggest there was not long-term stability in temperament estimates from early infancy to childhood, except in positive affective response to the environment and rate of recovery from distress and excitement. Measurement issues related to using the IBQ/CBQ, assessing temperament at an early age, maturational effects, and implications for practice are discussed.