In the health and life sciences and many other scientific disciplines, problem solving depends on mathematical skills. However, significant deficiencies are commonly found in this regard in undergraduate students. In an attempt to understand the underlying causes, and to improve students' performances, this article describes a ten-year survey (2000-2010) of mathematical skills of undergraduate Spanish students enrolled on a Plant Physiology course. The results show that the percentage of correct answers decreased for questions requiring some mathematical skills, particularly those needing calculations. Interestingly, percentages of failure do not explain the differences between correct answers for mathematical and non-mathematical questions. These suggest weaknesses in students' mathematical competency and also to a lack of self-efficacy. Additionally, although mathematical questions represented only about 14% of all the questions and did not show higher discrimination coefficients (0.27 versus 0.29), they were good predictors of the students' final grades. (Contains 4 figures, 1 box, and 1 table.)