Aim: An improved understanding of the relationship between radial and axial rigdity values would enable better appreciation of the clinical usefulness of RigiScan TM , the most widely utilized determination of erectile rigidity testing. Previous studies have shown that axial rigidity (measured by buckling forces) correlated well with radial rigidity (measured by RigiScan TM ) for radial rigidity values below 60%. For radial rigidity exceeding 60%, there was poor correlation. Heretofore, there has been no physiologic explanation of this phenomenon. Methods: During dynamic pharmacocavernosometry in 36 impotent patients, we investigated the relationship between axial buckling forces and RigiScan TM radial rigidity and, for the ®rst time, how they both vary with pressure, (which we varied over over a wide functional range). In addition, we recorded multiple penile length and diameter values enabling us to relate, also for the ®rst time, axial and radial rigidity to individual mechanical erectile tissue and penile geometric properties. Results: Marked differences were found in the manner RigiScan TM radial rigidity units and axial buckling force magnitudes increased with increases in intracavernosal pressure values in each individual. The former asymptotically approached a maximum ®nite value while the latter increased continuously towards in®nity. Based on data in this study, RigiScan TM radial rigidity values greater than 55% may be considered a necessary criteria for vaginal intromission capability in all partners but it is not a suf®cient one. Conclusions: Axial and radial rigidity share a common dependency upon intracavernosal pressure, however, they are also dependent upon other unique physical determinants. For axial rigidity, additional dependent variables include cavernosal erectile tissue properties and penile geometry, while for radial rigidity, this may include tunical surface wall tension properties. Clinical devices which assess functional penile rigidity should utilize axial and not radial rigidity testing.