It is shown that the parasitic radiation arising from a KRATKY-type collimator for the investigation of the small-angle scattering, also under ideal conditions, is caused by the diffraction of the x-rays on the edges of the slits. Calculations for the relative intensity of the parasitic scattering, using the FRESNEL-diffraction on half-planes and slits, give values approximately ten times greater than the measured values. A refined model leads to a better approximation. Analogues considerations for other collimators are given. The diffraction as a physical cause for the parasitic radiation yields principal limits for the measurement of the small-angle scattering.