A non-compliant high strength balloon (10) includes a reinforcement member (30) formed of a sleeve of woven or braided strengthening elements (32, 34). There are provided circumferential strengthening elements (32) which are pre-tensed in order to remove any undulations within the strengthening elements (32) and to ensure that these have a precise annular size. On the other hand, the longitudinal strengthening elements (34) have undulations therein. The pre-tensing of the strengthening elements (32) ensures that the balloon will not stretch during pressurisation thereof, which can occur when the circumferential strengthening elements have undulations therein, which will be natural artefacts of a weaving or braiding process. The undulations in the longitudinal strengthening elements (34) provides the balloon with increased longitudinal flexibility.
