The suitability of an ultra-accelerated concrete prism expansion test (testing concrete mixes in alkaline solutions at 150°C) for the assessment of alkali-silica reactivity (ASR) was investigated using concrete mixes made with different alkali levels and different typologies of Italian natural aggregates of known field performance. The results of previous concrete prism expansion tests at 38°C and 100% RH were used for comparative purposes. It was found that the ultra-accelerated test is appropriate for discriminating between innocuous and potentially alkali-reactive concrete mixes and may also be used as a rapid method for evaluating the threshold alkali levels of ASR-susceptible aggregates. The results of the ultra-accelerated test may be interpreted on the basis of a percent expansion criterion (0.12% at 21 days) or through the use of a reactivity parameter, ln k, the latter being determined from the correlation of the test results with a kinetic-based method. A value of -3.4 for ln k appears to be a suitable reactivity criterion for interpreting the results of the ultra-accelerated concrete prism test.