Recrystallized grainsize has been measured in quartz and, in some cases, feldspar, in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings: 1. (1) supracrustal, low-temperature, shallow dipping thrusts; 2. (2) deeper level major (> 100 km) thrusts in high-grade metamorphic rocks; and 3. (3) steeply dipping fault zones in granite within a low-grade Palaeozoic fold belt. The concept of a steady-state dynamically recrystallized grainsize largely unaffected by post-creep processes (e.g. static grain growth) receives strong support, especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones. However, the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize. The available theoretical and empirical stress/ grainsize relationships do not account for these effects. The concept of a steady-state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation. Predicted deviatoric stresses have been computed using Twiss' theoretical model for comparison with other crustal stress estimates, and most samples fall in the range from 20 to 150 MPa. In a few cases where discrete high-strain (mylonite) zones occur within contemporaneously regionally deformed rocks, the recrystallized grainsize indicates that stresses were 1.5 to 2 times higher within the zones. There is a clear need for improved theoretical and/ or empirical stress/grainsize relationships for quartz and other minerals before extensive further application of this technique.