A technique to cancel the frequency shift due to cold collisions of la
ser-cooled atoms is proposed. Results for future clocks based on Cs-13
5 and Cs-137 are presented. We find a Cs-137 fountain clock can be ope
rated with no cold-collision shift by adjusting the population ratios
in the fountain, and it may also be possible for Cs-135. This techniqu
e can also be applied to other phase-sensitive measurements using cold
atoms, including atom interferometers.