Bacterial biofilms on permanent catheters are the major sources of infection. Exposure to ultraviolet-C (UVC) light has been proposed as a method for disinfecting the inner surface of catheters. Specification of a UVC-based device for in vivo disinfection is based on the knowledge of the required doses to kill catheter biofilm. Given these doses and the power of available UVC light sources, calculation of the necessary treatment times is then possible. To determine the required doses, contaminated urinary catheters were used as test samples and UVC treated in vitro. Patient catheters (n = 67) were collected and cut into segments of equal size and treated with various UVC doses. After treatment, the biofilm was removed by scraping and quantified by counting colony forming units. Percentage killing rates were determined by calculating ratios between UVC-treated samples and controls (no UVC treatment). Mean killing rates were 89.6% (0.5 min), 98% (2 min), and 99% (60 min). Approximately 99% killing was obtained with a UVC dose of 15 kJ m(-2). This dose, which is about 100 to 1000 times greater than the lethal dose for planktonic cells, is expected to be the maximum dose required to maintain newly inserted catheters free of contamination. The combination of high doses required to kill mature biofilm and the limited effect of current UVC light sources result in a relative long treatment time (similar to 60 min). If a UVC-based method is to be of practical use for disinfection of catheters in the clinic, repeated preventive treatments should be carried out on newly inserted catheters.