Abstract: A multilumen unilimb breathing circuit with a detachable proximal fitting for providing respiratory gases to and receiving expiratory gases from a patient. The circuit has unique fittings for connection to patient devices or assisted ventilation systems and components. Despite many teachings away in the prior art from making breathing circuit tubing detachable from the proximal terminal, the proximal end of the rigid multilumen proximal fitting can be attached and detached by a user at a site of use to a mating multilumen proximal terminal while its distal end can be attached to multiple lumens formed of flexible tubing for carrying gases to and from a patient. The present invention introduces for the first time to the multilumen unilimb circuit art a detachable proximal fitting.