A medical device such as a stent (10) or medical balloon (40) is at least partially coated with a saturated carboxylic acid layer in order to enhance biocompatibility, reduce thrombogenesis and increase endothelialisation. The coating is preferably of citric acid in non-crosslinked form and preferably non-porous so as to mask the underlying structure of the medical device. The acid coating forms an outer surface of at least a part of the medical device, that is has no other layer or material overlying it, save for in some embodiments a partial coating of a bioactive material.