Using electron microscopy, we prospectively evaluated how frequently adherent microorganisms colonized silicone rubber intravenous (Hickman) catheters removed from patients with cancer. Thirteen (87%) of 15 catheters had gram-positive cocci in glycocalyx adherent to the surface of the catheter lumen. Fungal elements or gram-negative bacilli were mixed with the gram-positive cocci in the glycocalyx on the lumens of three catheters. A consistent morphologic form was adherent to, and the same species was recovered from, the corresponding catheter for six of 27 organisms causing septicemia during catheterization: four of five Staphylococcus epidermidis bacteremias and the only Staphylococcus aureus bacteremia, and one of five candidemias. Three of these six septicemias were successfully treated without removal of the catheter. Although adherent organisms, particularly S epidermidis, were likely to be present on the surface of the lumen of long-term, indwelling, silicone intravenous catheters, septicemias potentially related to these organisms occurred infrequently (fewer than two per 1000 days of catheter use), and the suspect septicemias could sometimes be treated without removal of the catheter.