A cryogenic tissue ablation instrument includes an elongate flexible body with a proximal supply port for coupling with a pressurized coolant, a supply lumen in communication with the proximal supply port, and an expandable balloon carried on the elongate body. A dispersion member coupled to the elongate body has an interior lumen in communication with the supply lumen, the dispersion member having one or more coolant dispersion apertures sized and located such that a pressurized flowable coolant will enter the balloon interior in the form of a liquid spray that contacts and provides substantially uniform cooling of the balloon.