Prediction of the penetration rate (PR) of horizontal directional drilling (HDD) depends almost entirely on the experience of practitioners. This has resulted in actual values of PR that are inconsistent with the predicted ones. The problem stems from the many factors that are involved in the process and the need to assess their relative importance. To determine the factors that significantly affect PR and their relative importance, semi-structured interviews were conducted with HDD experts and a questionnaire was sent to other experts worldwide. The obtained responses are presented and discussed in the current paper. It has been found that ten factors significantly affect PR. The highest three factors are ‘ground type and condition’, ‘geotechnical investigation’, and ‘operator experience’. The two factors ‘geotechnical investigation’ and ‘steering engineer experience’ which are newly introduced in this study are ranked as the 2nd and 5th most important, respectively. The interrelationships between factors are also discussed. The obtained findings show reasonably good comparison with observations of actual HDD works. On the basis of Pareto analysis made on the obtained responses, it has been found that the top 10 factors represent 72% of the effect on PR. In addition, implications for actual HDD works are presented