Fifty-three consecutive subclavian or jugular hemodialysis catheters inserted into 41 patients were prospectively studied over a period of 8 months in order to determine the incidence of infection and its mechanisms. The intravascular, intradermal and the Y catheter segments as well as both connections were cultured using a quantitative technique for the intraluminal surface. In addition, the intravascular and intradermal portions of the catheter were cultured using a semiquantitative technique for the external surface. Skin smears of the catheter entry site were also cultured, and blood cultures were similarly obtained if fewer developed. Twenty-nine of the 53 catheters (55%) were significantly colonized by one (19 cases) or more (10 cases) microorganisms. The source of the colonizing microorganisms was the skin in 17 cases (58%), intraluminal in 5 (17%), both routes in 5 (17%) and others in 2 (6.8%). Staphylococcus epidermidis (22 cases) and Staphylococcus aureus (4 cases) were the bacteria most frequently isolated. Nine of the 53 catheterizations (17%) were complicated by catheter-related septicemia due to S. aureus in 4 cases, S. epidermidis in 3 cases, Streptococcus faecalis in 1 and Proteus vulgaris in 1. Catheter-related bacteremia contributed to a patient's death in 1 case. Suppurative local infections of the catheter entry site developed in 3 cases, 2 of them with septicemia. We conclude that the rate of infection due to subclavian or jugular hemodialysis catheters is very high and that the skin is the most frequent origin of the microorganisms.