Axial rigidity is the most important characteristic of a functional erection. Three factors determine axial rigidity: intracavernosal pressure (addressed by hemodynamics), cavernosal tissue mechanical properties ("tissue expandability"), and the ratio between the two dimensions of the penis: length and circumference ("penile geometry"). Intracavernosal pressure only is addressed by hemodynamic diagnostics, dynamic cavernosometry being the only investigation that allows its direct determination. The appreciation of these three rigidity determinants has the implication that there might exist cases of erectile dysfunction for pure geometric reasons, i.e. an excessive erect length/circumference ratio (L/C R). We report our series of 57 consecutive dynamic cavernosometries with concomitant axial rigidity and penile dimensions recording. Six cases of purely "geometric erectile dysfunction (ED)", i.e. patients unable to physiologically develop a rigid erection for a pathologic L/C R were identified. These patients are characteristically young men (mean age: 28 yrs, range 22-33) with life-long ED not responding to oral or intracavernosal treatment. Our L/C R cut-off value for pure geometric ED is 1.31. We also report two surgical strategies that could possibly address pure geometric ED. In conclusion, hemodynamics is not sufficient to assess the potency status, as other factors come to determine penile rigidity. In particular, there are cases of pure geometric ED which, potentially, is surgically curable. These cases should not be erroneously labelled as psychogenic cases.