Internal friction measurements were made as a function of hydrostatic pressures up to 3 kb by use of a piezoelectric torsional oscillator inside the pressure vessel, as well as with a Marx composite oscillator both before and after pressure treatment. Specimens were prepared by cold‐working 99.99% aluminum polycrystals to a 50% reduction in width, followed by an anneal at 300°C. Measurements outside the pressure vessel showed that the strain amplitude dependence was irreversibly increased by amounts up to 1500% of the initial values as a result of the application of 3 kb for 15 min at 25°C. The original amplitude dependence was restored by annealing 15 min at 300°C. A subsequent compression again increased the amplitude dependence but to a lesser extent. After several such annealing‐compression cycles there was no irreversible effect within 4% of background. Additional cold‐work followed by annealing again produced the effect. The effect increased with increasing pressure. An increase in amplitude dependence of up to 600% also occurred in samples which had not been previously cold‐worked. The internal friction at constant strain amplitude increased faster than linearly with increasing pressure and decreased linearly with decreasing pressure. Successive pressure cycles gave the same effect but with decreasing magnitude. After about 3 cycles, both increasing and decreasing pressure gave the same linear dependence of the damping, corresponding to within 8% of the background damping. It is suggested that irreversible dislocation effects might occur as the result of shear stresses produced near voids by the hydrostatic stress.