Aims/hypothesis. We aimed to find if the relation between insulin sensitivity and beta-cell function assessed from fasting and OGTT measurements has a physiological shape (hyperbolic with the reference methods). Methods. Healthy women without diabetic first-degree relatives underwent a 75 g OGTT with plasma glucose and insulin (n = 35) concentrations being measured at 0, 30, 60 and 120 min. Beta-cell function and insulin sensitivity were estimated using previously described indices from fasting (1 for beta-cell function, 6 for insulin sensitivity) and OGTT measurements (3 for beta-cell function and 5 for insulin sensitivity). A hyperbolic relation was tested for the 21 beta-cell function-insulin sensitivity pairs using a non-lineal regression method. Results. The assessment of beta-cell function from OGTT was impossible in seven women and one had outlier indices. For the remaining 27 women, only 8 combinations adjusted to a hyperbolic relation. The best adjustment was achieved using the fasting glucose to insulin ratio as the estimation of insulin sensitivity and the homeostasis model assessment (HOMA) index (single fasting sample) as the estimation of beta-cell function (r ² 0.802, k 869.71, p < 0.001). Conclusion/interpretation. In this group of healthy women, the estimation of insulin sensitivity and beta-cell function by most methods using OGTT-derived glucose and insulin measurements did not adjust to a hyperbolic relation but all fasting indices combinations did. Beta-cell function estimated with the HOMA index and insulin sensitivity with fasting glucose to insulin ratio had the best adjustment. [Diabetologia (2000) 43: 1507–1511]