Recent research has shown that personality traits continue to develop throughout the life span, but most profound changes are typically found during young adulthood. Increasing evidence suggests that life events play a significant role in many of these changes. The present longitudinal study examined the role of work, education, social, and health-related life events in the development of the Big Five traits among young Finns. Participants were originally recruited in 2004 through elementary schools in a middle-sized Finnish city. Participants' Big Five traits and life events were measured via self-reports at ages 20 and 23 (Ns = 597 and 588, respectively). Entering work life, beginning a relationship, and studying in university predicted increases in Conscientiousness, trying drugs predicted increases in Neuroticism, and onset of a chronic disease predicted increases in Neuroticism and Conscientiousness between ages 20 and 23. The results suggest that mature life transitions relate to stronger increases in Conscientiousness in young adulthood, and that non-normative life choices and events may predict increases in Neuroticism.