Method and apparatus for reporting feedback information for multi-carrier operation are disclosed. For more effective support for the asymmetric traffic loads in the downlink, a wireless transmit/receive unit (WTRU) is configured with multiple carriers with an unpaired downlink carrier(s). The unpaired downlink carrier is an active downlink carrier that does not have a corresponding active uplink carrier. The WTRU reports feedback information for multi-carrier operation including feedback for the unpaired downlink carrier. For transmission of the feedback information for the unpaired downlink carrier, a feedback channel may be allocated on a distinct non-overlapping resource region on the uplink carrier so that the network may determine which downlink carrier the received feedback information is for based on the resource region. Alternatively, a different feedback channel may be allocated for the unpaired downlink carrier. Alternatively, the feedback information may be transmitted via medium access control (MAC) encoded feedback.