During a one-year period, 236 intravascular catheter tips were investigated by culturing the outside by rolling the tips on the surface of blood agar plates, the inside by flushing with nutrient broth, and finally by culture after ultrasonication. Clinical information was collected prospectively by visiting the ward and symptoms and signs of catheter infection were correlated with the results of the catheter cultures. Coagulase-negative staphylococci were the organisms that were most frequently isolated from the catheters. The results of culture showed a clear bimodal distribution, and counts of 100 colony-forming units (cfus) and culture from the inside of the catheters were the best predictors of infection. Almost all infected catheters were colonized both on the inside and outside.