INTRODUCTION: Students' formal networks, which are formed by a formal curriculum design, such as formally organized study groups within learning communities (LCs), may benefit students' interactions and learning. It is unclear how large-scale LCs contribute to the formation of different informal peer relationships, which refers to student self-organized out-of-class relationships. Two mechanisms can explain relationship formation in LCs. Propinquity within formal networks and homophily of students' characteristics (nationality, sex, academic performance) may promote students' peer relationships. This study explores to what extent the formation of students' informal networks was determined by their formal networks (LCs) while controlling for students' characteristics and which mechanisms play an important role. METHODS: With online surveys, data were collected about five informal networks (help-seeking, collaboration, information sharing, friendship, and learn-from) from 69 first- and 51 second- bachelor year medical students (2890 relationships). Students were divided into four LCs in the formal curriculum. We compared students' five informal network structures between first- and second-year students, domestic and international students, within and between formal networks. Besides, we used Quadratic Assignment Procedure (QAP) Regression Analysis in Ucinet to investigate the associations between students' informal and formal networks (LCs) and students' characteristics. RESULTS: Propinquity (in the same LC) plays a role since students have more informal connections within LCs than between LCs. Furthermore, it seems to play a greater role for second-year students than for first-year students. Homophily of nationality is important in informal networking since students are more likely to connect with others of similar nationalities. CONCLUSION: Students become more connected within the LC when they remain in the same LC for a longer period. Formal networks enhance the students' informal interactions within LCs but seem to restrict the interactions among students from other LCs. International students need support in order to integrate with domestic students in LCs.