This study examines the association of stress with performance and health in first-year dental students and changes in the amount and sources of stress over one year. Students at four U.S. dental schools completed the Dental Environment Stress (DES) scale, Perceived Stress Scale (PSS), stress rating, and demographic questions at the start of their first year of school (baseline), 11.7 weeks, and one year later when first-year GPA, illnesses, health ratings, and symptom frequency were also assessed. Overall, 296 (186 men, 110 women) responded at baseline and after one year; 205 responded all three times. Stress scores were negatively correlated with GPA (DES, p=.006; PSS, p=.04; stress rating, p=.002) and with physical and emotional health ratings (p's< or =.002), but positively associated with illness (p<.05), symptoms (p<.0001), and symptom frequency (p's<.05). Stress was higher after one year (p's<.001) and varied by school (p<.0001). Women (p<.01), younger (p<.003), and single students (p<.03) had higher stress at baseline, but after one year, there were no differences by gender, age, or marital status. Ratings for items on the Dental Environment Stress scale related to schoolwork were high at baseline and increased further by one year (p's< or =.0001); items related to school atmosphere had low ratings initially with large increases over time (p's<.0001). In conclusion, stress increases over time in first-year dental students and is related to detrimental effects on performance and health. Variation between schools may reflect different teaching methods. Changes in sources of stress may reflect the different contributions of anticipatory and situational stress over time. First-year dental students may benefit from stress reduction programs.