Effectiveness of prefetch instructions is determined. A prefetch instruction is executed to request that data be fetched into a cache of the computing environment. The effectiveness of the prefetch instruction is determined. This includes updating, based on executing the prefetch instruction, a cache directory of the cache. The updating includes, in the cache directory, effectiveness data relating to the data. The effectiveness data includes whether the data was installed in the cache based on the prefetch instruction. Additionally, the determining the effectiveness includes obtaining at least a portion of the effectiveness data from the cache directory, and using the at least a portion of effectiveness data to determine the effectiveness of the prefetch instruction.