A method of manufacturing a balloon portion for a balloon catheter for radially expanding a vessel in the body of a mammal, which catheter comprises a tube portion with a passage therethrough and a hollow expandable balloon portion defined by a fluid impervious wall and secured to the tube portion, which balloon portion can be inflated and deflated by means of a fluid passed through the passage, such that: a. the wall of the balloon portion is formed from a flexible substantially fluid impervious material having reinforcing fibers formed integrally with the wall material; and b. the balloon portion is preformed to the desired radial diameter at its inflated state having smaller diameter end portions and a wider diameter portion intermediate the said ends and has a substantially uniform wall thickness.