This paper summarizes extensive, pioneering experience with the first 5-year well-testing project in the hostile ultradeep wells of the Villafortuna-Trecate field. Performance of the various downhole high-temperature, high-pressure (HTHP) gauges used is investigated, and the relevant technology is compared throughout the field history. Awareness of the quality of data acquired and the role played by redundancy (use of more gauges simultaneously) are identified as key points in planning successful future HTHP operations.