Passive mode-locking relies critically on a saturable loss mechanism to form ultrashort pulses. However, in Kerr-lens mode-locking (KLM), no actual absorption takes place, but rather losses appear due to diffraction, and actual light must escape the cavity. The Kerr-lens effect works to generate through diffraction an effective instantaneous saturable absorber that depends delicately on the interplay between the spatial and temporal profiles of the pulse. Despite the importance of KLM as a technique for generating ultrafast pulses and the fundamental role of diffraction losses in its operation, these losses have never been observed directly. Here, we measure the light that leaks out due to diffraction losses in a hard-aperture Kerr-lens mode-locked Ti:sapphire laser, and compare the measured results with a numerical theory that explicitly calculates the spatiotemporal behavior of the pulse.