Objectives: The ability of medical students to form positive relationships with patients is influenced by the students’ personality.The psychological characteristics necessary for effective physician-patient interactions (PPIs) have not been identified. Methods: In August of 2012, 2013, and 2014, second-year medical students at Gyeongsang National University completed the Korean version of the Defense Style Questionnaire (K-DSQ). Following completion of their 2-year clerkships, the students completed the KDSQ again. Of the 216 second-year students, 187 (94 males and 93 females) aged 24-38 years (mean, 28.9±2.8 years) completed the K-DSQ at year 4. PPIs were rated by standardized patients during practical examinations following the clerkships. Results: Female sex (β=-0.157, p=0.035) and a more immature defense style in year 2 (β=0.197, p=0.027) were significant predictors of better PPIs in year 4. An increase in humor during the clinical clerkship (β=0.171, p=0.030) and female sex (β=-0.170, p=0.023) were significant predictors of the year-4 PPI ratings. Conclusion: Female sex and a more immature defense style in the second year, as well as an increase in humor during clerkships predicted positive PPIs.