Existing coaxial balloon catheters cannot be passed through the smaller catheters used for superselective angiography; nor can they be used by themselves for selective catheterization, as the balloons are easily damaged. The authors describe the principles and construction of double-lumen catheters ranging from 0.45 to 0.85 mm in outer diameter, which may be passed through 4.1-6.0 F catheters to engage smaller vessels. Double-lumen catheters with an outer diameter of 0.45-0.56 mm permit injection of ethanol or a mixture of isobutyl-2-cyanoacrylate, iophendylate, and tantalum powder. The 0.85-mm size is best suited for low-viscosity silicone rubber opacified with tantalum.