The idea of a diagnostic process to identify students likely to benefit from additional support originated from two separate investigations relating to the performance of entry-level mathematics students at the University of South Africa (UNISA). One project explored the relationship between reading skill and mathematical performance. This resulted in a reading intervention programme for students of mathematics. The other project considered an alternative method of assessment. This article explains why and how diagnostic assessment was developed for students who enrolled for the UNISA Mathematics Access Module. The article gives some initial results, and an analysis of examination performance related to the risk categories identified by the diagnostic assessment. (Contains 2 figures, 8 tables and 5 notes.)