Two studies investigated the importance of initial topic interest (i.e., expectation of interest) and tutors' autonomy-supportive or controlling instructional styles for students' motivation and performance in problem-based learning (PBL). In Study 1 (N = 93, a lab experiment), each student participated in a simulated group discussion in which tutor instructions were manipulated to be autonomy supportive, internally controlling, or externally controlling. Controlling tutor instructions led to higher controlled motivation, but autonomy-supportive instructions did not relate to students' autonomous motivation and performance measures. Higher topic interest resulted in higher autonomous motivation and contributed indirectly to more self-study time and persistence. Self-study time was in turn associated with better test performance. A field study (N = 287, Study 2) supported the findings of Study 1. Tutor-provided autonomy support was unrelated to autonomous motivation, while a controlling instructional style led to higher controlled motivation and negatively affected performance. Again, topic interest positively influenced autonomous motivation and subsequent performance. Both studies demonstrate the importance of students' initial topic interest for subsequent performance in PBL. Results also indicate that in PBL, tutor-provided autonomy support does not improve autonomous motivation and performance, whereas controlling tutoring can promote controlled motivation and hamper performance. Implications and further research opportunities are discussed.