The aim of this study was the quantitative evaluation of the myotatic reflex in a group of 26 patients affected by stationary spastic paresis (6: hemiparesis; 5: paraparesis; 8: tetraparesis; 7: multiple sclerosis) before and after a treatment of hydro-kinesy therapy. The treatment was carried out in an indoor pool containing warm (32°C) sea water and consisted of active and passive motion exercises, coordination exercises and immersion walking. The measured parameters were: (i) the peak input force (FpH) measured by means of an instrumented hammer with which the patellar tendon was hit; and (ii) the peak value of the corresponding reflex force of the quadriceps femoris (FpQ) measured by means of a load cell connected to the subject's ankle. The peak values of the reflex response (FpQ) were found to increase as a function of the intensity of the imposed stimulus and to reach a plateau between 15 and 30 N of FpH. A Student's t test applied to the paired values of FpQ (as measured at plateau conditions) on both the lower limbs, before and after therapy, showed no significant changes due to the treatment in the four groups of subjects. However, if all subjects were grouped regardless the type of illness: 1) the average reflex response of the affected limb (the one characterized before therapy by the higher FpQvalues) was found to decrease following the treatment (75.1 ± 26.7 N pre therapy and 69.1 ± 29.3 N post therapy, p = 0.07, n = 26); and 2) the effect of the treatment was found to be significantly larger (p = 0.04, n = 26) on the affected limb (Δ FpQ = 6.07 ± 16.5 N) as respect with the contra lateral one (Δ FpQ = −0.16 ± 12.1 N).