An 18 month prospective study of personality traits (as assessed by Cattell's Children's Personality Questionnaire and Sixteen Personality Factor Questionnaire) was conducted on twin pairs and their parents. The primary personality trait scores for both the adults and children had relatively low stability over the 18 month interval; in addition, in some cases, the inter-factor stability coefficients exceeded the intra-factor coefficients. This finding, coupled with the lack of parent/offspring similarity for the primary personality traits, places in question the predictive power of Cattell's personality questionnaires. A factor analysis on the original items indicated that 5 oblique factors for the children and 6 oblique factors for the adults need be retained. These factors were not only more stable than the primary trait scores, but they also did not have such high inter-factor stability coefficients. Except for Neuroticism and IQ, the adult and juvenile factors do not show structural similarities.