In the health and life sciences, as well as in 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' performance, this paper reports a ten-year survey (2000-2010) on mathematical skills of undergraduate Spanish students enrolled in a Plant Physiology course, based on data from tests performed throughout this time period. The results show that the percentage of correct answers decreases for questions requiring some mathematical skills, particularly those needing some calculations. Interestingly, percentages of failure are similar for both types of questions. These results suggest not only the existence of weaknesses in students' mathematical competency but also 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 demonstrated to be a good predictor of the students final grade.