There is a hyperbolic relationship between insulin sensitivity and beta-cell function. The product between an individual’s insulin sensitivity and beta-cell function defines an area that actually corresponds to his/her underlying beta-cell function adjusted to his/her prevailing insulin sensitivity. Hence, the hyperbolic product is likely to better reflect an individual’s beta-cell function, and therefore the natural history of glucose homeostasis, including the normal glucose tolerance (NGT) to impaired glucose tolerance (IGT) and IGT to type 2 diabetes (T2DM) transitions, as well as the progressive requirement for hyperglycaemia therapies. To examine whether the product of insulin sensitivity and insulin secretion in the fasting state might be a useful measure to predict progression from NGT to impaired fasting glucose (IFG) and from IGT to type 2 diabetes, a cohort of 300 subjects with NGT and 75 subjects with IGT were followed up over 5 years, insulin sensitivity being calculated using the Belfiore index (B) and insulin secretion by the HOMA-beta cell index at regular intervals. The product of B-beta is expressed as: (40 x Ins(0) pmol L(-1))/Glu(0) mmol L(-1){[(Glu(0) mmol L(-1)x Ins(0) pmol L(-1)) + 1] - 3.5[(Glu(0) mmol L(-1) x Ins(0) pmol L(-1)) - 1]}, where Glu(0) is fasting glucose and Ins(0) is fasting insulin. This product of B-beta predicted the progression from NGT to IGT (RR=2.7; CI 95% = 1.2-9.1) and from IGT to T2DM (RR=5.3; CI 95% = 1.3-8.55); cut-off points for the B-beta product that better predicted progression from NGT to IGT, and from IGT to T2D were 0.25 and 0.15, respectively. Thus, this simple measurement could be used in clinical practice to track the worsening of glucose metabolism.