An adjustable balloon catheter (21) for extravasated drug delivery is disclosed generally comprising an outer catheter (24) with a first balloon (28) and an inner catheter (26) with a second balloon (30). The inner catheter is moveably disposed in a lumen of the outer catheter such that the inner catheter moves relative to the outer catheter, changing the distance between the balloons. A fluid source (22) supplies air to the catheters via lumens therein to inflate the balloons to create a chamber (64) between them, and at least one of the catheters has a lumen (44) for delivering a therapeutic and/or diagnostic agent to the chamber. The inner catheter is then moved relative to the outer catheter to decrease the size of the chamber between the balloons and decrease the fluid pressure therein such that the therapeutic and/or diagnostic agent is extravasated into the surrounding tissue. In some cases, the inner catheter and its balloon can be inserted through the lumen of the outer catheter.
