In the mathematics education literature, subject matter knowledge is well recognized as the cornerstone of a mathematics teacher's knowledge base. Though the 'Mathematics Problem', the declining standards of students' mathematical abilities upon entering tertiary education, is well documented in many countries, its effect on preservice mathematics teachers has not yet been analyzed to date. Therefore, we analyze 17 years of data (1997-2013) from a mathematics diagnostic test taken by preservice post primary mathematics teachers (n = 365) during their first mathematics class in a teacher training institution. Our analysis shows that the mathematical knowledge of preservice teachers has declined in all of the assessed core areas of arithmetic, algebra, geometry and calculus, with the more recent years of 2007-2013 showing more rapid decline than the years before. In fact, the average-performing preservice teacher in 1997 achieved a higher score than almost all preservice teachers in more recent years.