Career maturity is defined as an individual's readiness to make a career decision. The aim of this study is to examine how personal characteristics and environmental factors are connected to the career maturity of high school students. 168 male and female students attending the fourth grade of a gymnasium or vocational school in Croatia participated in an anonymous online survey. The personal characteristics we studied were gender, grade point average (GPA), personality traits and interests. The results showed that female students are more professionally mature than male students and that GPA and personality traits (conscientiousness, agreeableness, neuroticism and openness to experience) were positively correlated with career maturity, but interests weren't. The environmental factors we included were parental behaviors (support, interference and lack of parental involvement) and school system support (a professional service or a headroom teacher). Career maturity was positively correlated with the subscale of parental support but negatively correlated with the subscales of parental interference and lack of parental involvement. The correlation between perceived support of the professional service and career maturity was low, while insignificant with the perceived headroom teacher's support. Hierarchical model of personal characteristics and environmental factors is significant in predicting the career maturity of high school students. Environmental factors explain 9% of variance of professional maturity over and above personal characteristics.