Medical device balloons are formed from a tubular parison by a process or apparatus which establishes a controlled location (initiation zone) on the parison where radial expansion is initiated. Initiation within the initiation zone is achieved by heating the parison in that location to a higher temperature than the remainder of the parison for at least a portion of the blowing time. A variety of apparatus configurations are provided, some of which allow for the size and location of the initiation zone to be readily reconfigured. Balloons can also be modified, post-blowing, using heating apparatus and methods described.