A method and apparatus for dynamically allocating hybrid automatic repeat request (H-ARQ) processes in a wireless transmit/receive unit (WTRU) for supporting enhanced uplink (EU) transmissions. The H-ARQ processes in the WTRU are reserved for specific transport channels (TrCHs), dedicated channel medium access control (MAC-d) flows or logical channels associated with different data transmission priority classes. The WTRU allocates H-ARQ processes from those reserved H-ARQ processes that are available. Optionally, a higher priority channel may be allowed to allocate an H-ARQ process reserved for lower priority channels. Lower priority H-ARQ processes may be preempted. The preemption may be restricted by urgency of data transmission, or by radio network controller (RNC) configuration of H-ARQ processes. Alternatively, a common pool of H-ARQ processes may be configured and an H-ARQ process may be allocated from the common pool in accordance with a priority of each channel, and lower priority H-ARQ may be preempted.