The self-retaining balloons of the silicone catheters (Dover and Travenol) will losefluid according to the concentration of the solutions injected. The process is temperature-dependent and must be governed by a balance between hydrostatic and osmotic pressure gradients. Where loss of fluid from a low-volume balloon may be critical in the premature loss of an indwelling silicone catheter, we recommend that a concentration equivalent to 5 per cent saline be used in the balloon.