The configuration factor of adhesive cavities is defined as the ratio of the restoration's bonded to unbonded (free) surfaces. Such a configuration factor was described, on ideal cavities, as having a potential value in predicting the behaviour of the restorations, because it is related to the restoration's capacity for relieving stress by flow. The aim of this study was to measure the configuration factor value for real Class I and II cavities and simulated cervical erosions prepared in molars. Ten Class I, five Class II cavities and seven cervical erosions were analysed using a computerised digitising system. The configuration factor values found were 4.03 +/- 0.33 for Class I cavities, 1.85 +/- 0.59 for Class II cavities and 1.10 +/- 0.09 for simulated cervical erosions (P < 0.01).