Kidney lithotripsy patients frequently receive epidural anesthesia via indwelling epidural catheters. In our hospital, patients are immersed in a tub of warm, continuously-flowing tap water. The epidural catheter-entry site is covered by a transparent occlusive dressing. To determine the risk of microbial colonization of the epidural catheter during lithotripsy, we performed quantitative cultures of tub water and semiquantitative cultures of catheters in 63 lithotripsy procedures. Most of the tub water organisms were typical tap water and skin flora isolates. Total colony counts were generally low with no significant progression during the course of serial procedures. Forty-two epidural catheters were cultured; 34 (81%) were sterile, 8 (19%) were colonized with small numbers of flavobacteria or coagulase-negative staphylococci. Only four catheters had organisms present on catheter segments covered by the transparent occlusive dressing (in each case there was a single colony forming unit per semiquantitative plate) and these organisms were probable contaminants. We conclude that with our current lithotripsy procedures, the risk for the development of epidural catheter-associated infection seems to be low.