Acoustic emission detection has been used to study the damage occurring in unidirectional carbon-fibre-reinforced plastics (CFRP) during cyclic loading and stress relaxation at high stress levels. The results suggest that true fatigue processes do not occur in CFRP and that the damage sustained during cycling is of the same kind as that which occurs in ordinary tensile loading. As a result either of many reversals of load or of holding a sample at constant load, the rate of emission of stress waves associated with fibre fractures diminishes with time until the sample becomes silent. A sample in this state is relatively safe from further damage, and a procedure of proof testing based upon this result is suggested.