Abstract The rates of nucleation and growth of crystals in ZBLAN glass have been analysed and differences have been found between previously reported models. These differences are most acute in the case of crystal nucleation rates. It is shown that, depending on which model is selected, the predicted stability of the glass is dramatically altered. The lowest stability predictions appear to have been based upon the most reliable data. The more reliable nucleation and growth models are used in a computer simulation which predicts crystal size distributions for any chosen thermal cycle. The thermal cycles arising from preform casting and fibre drawing are considered, and the resulting distributions are presented. Signal losses due to scattering of between 0.1 and 10 dB/km at wavelength 2.55 μm are predicted for long optical fibres, assuming that the nucleation and growth models are accurate. The predicted loss is sensitive to the time and temperature of the fibre drawing process, and is shown to be lower for a fast hot draw than for a slower cooler draw.