The validity of electron‐density determinations using spherical probe in medium pressure discharge plasmas (rp/&lgr;e∼0.3 to 40, whererp=probe radius, &lgr;e=electron mean‐free path) are examined for density rangeNe=109−1011cm−3. Various collision‐dominated probe theories are used to obtainNefrom both electron and ion saturation currents (IeandIi).Nedetermined fromIecoincide reasonably with those determined from microwave measurement, whileNefromIiare always greater than those from microwave measurement by a factor of 2 to 100. The discrepancy increases with increasing gas pressure and total discharge current, andIican not be used forNedeterminations. There is ``break'' on the ion current characteristics which is the same as that reported in the measurement of very low‐density flowing afterglow plasmas (Ne<108cm−3). Explanation is offered in connection with the unusual increase of ion current caused by the release of electrons from the probe surface by collisions of metastables and the decrease of regular thermal‐motion current because of the short mean‐free path condition (&lgr;6<rp).