The impact of optical feedback on the direct performance of a monolithic InAs/GaAs quantum dot passively mode-locked laser intended for applications such as multigigahertz interchip/intrachip clock distribution is experimentally investigated. Evaluation of the feedback resistance is an important feature, as the laser is to be monolithically integrated on chip with other devices, in which case optical isolation is difficult. This work shows that a feedback level on the order of −24 dB is detrimental for mode-locking operation, enhancing noise in the rf electrical signal, strongly narrowing the useful mode-locking region as well as causing central frequency shift, and severe instabilities.