Problem-based learning (PBL) is a constructivist approach to learning which is believed to promote reflective thinking in students. This study investigated how students in one particular institution developed in their reflective thinking habits--Habitual Action, Understanding, Reflection, and Critical Reflection--as they went through the daily practice of PBL. A 16-item questionnaire measuring the four levels of reflective thinking habits was administered to four cohorts of students: an incoming cohort, first-years, second-years, and third-years. First-year students rated themselves higher on Reflection and Critical Reflection, while third-years reported the highest levels of Habitual Action. Discriminatory and scatterplot analysis on the third year cohort revealed that while a proportion of students (47%) reported higher levels of Habitual Action with lower levels of Reflection, there was a small subgroup who also reported higher levels of both Habitual Action and Reflection. Overall, the results showed that PBL does promote the development of reflective thinking, particularly for first-year students, but that this development is not sustained consistently after that. This pointed to other possible factors that could hinder students' development of reflective thinking in PBL.