The rising number of newly insured young adults brought on by health care reform will soon increase demands on primary care physicians. Physicians will face more young adult patients, which presents an opportunity for more prevention-oriented care. In the present study, we evaluated whether brief observer reports of young adults' personality traits could predict which individuals would be at greater risk for poor health as they entered midlife. Following the cohort of 1,000 individuals from the Dunedin Multidisciplinary Health and Development Study (Moffitt, Caspi, Rutter, & Silva, 2001), we show that very brief measures of young adults' personalities predicted their midlife physical health across multiple domains (metabolic abnormalities, cardiorespiratory fitness, pulmonary function, periodontal disease, and systemic inflammation). Individuals scoring low on the traits of Conscientiousness and Openness to Experience went on to develop poorer health even after accounting for preexisting differences in education, socioeconomic status, smoking, obesity, self-reported health, medical conditions, and family medical history. Moreover, personality ratings from peer informants who knew participants well, and from a nurse and receptionist who had just met participants for the first time, predicted health decline from young adulthood to midlife despite striking differences in level of acquaintance. Personality effect sizes were on par with other well-established health risk factors such as socioeconomic status, smoking, and self-reported health. We discuss the potential utility of personality measurement to function as an inexpensive and accessible tool for health care professionals to personalize preventive medicine. Adding personality information to existing health care electronic infrastructures could also advance personality theory by generating opportunities to examine how personality processes influence doctor-patient communication, health service use, and patient outcomes.