This study gives an insight into the differences between student understanding of line graph slope in the context of physics (kinematics) and mathematics. Two pairs of parallel physics and mathematics questions that involved estimation and interpretation of line graph slope were constructed and administered to 114 Croatian second year high school students (aged 15 to 16 years). Each pair of questions referred to the same skill in different contexts--one question in the context of mathematics and the other in the context of kinematics. A sample of Croatian physics teachers (N = 90) was asked to rank the questions according to their expected difficulty for second year high school students. The prevalent ranking order suggests that most physics teachers expected mathematics questions to be more difficult for students than the parallel physics questions. Contrary to the prevalent teachers' expectations, students succeeded better on mathematics than on physics questions. The analysis of student answers and explanations suggests that the lack of mathematical knowledge is not the main reason for student difficulties with graphs in kinematics. It appears that the interpretation of the meaning of line graph slope in a physics context presents the largest problem for students. However, students also showed problems with the understanding of the concept of slope in a mathematical context. Students exhibited slope/height confusion in both contexts, but much more frequently in the context of physics than in the context of mathematics.