The presence of biofilm is thought to accompany infection or colonization of chronic peritoneal dialysis (PD) catheters, and to be an important pathogenetic factor in the recurrence or persistence of peritonitis. In the current study, the characteristics of identifiable biofilm associated with the PD catheter were studied using scanning and transmission electron microscopy in six consecutive cases requiring catheter removal for a variety of indications. Biofilm characteristics in each case were rated in blinded fashion by three independent observers, and findings were then correlated with the clinical histories and microbiologic findings. Surprisingly, two of the three cases with the most severe biofilm formation occurred in patients with no history or microbiologic findings of recent infection, and the positive findings of leukocytes, macrophages, fibrillar matrix, and other structures on these catheters did not correlate with detectable infection. In addition, extracellular spherical lipoid structures and intracellular lipoid vacuoles in mesothelial-like cells were prominent in four of six cases, did not correspond to the presence of infection, and suggested possible mesothelialization of the catheter. The findings of this study do not necessarily controvert the microbial origin of some components of the biofilm, or the possible role of biofilm in some cases of persisting peritoneal infection. However, it is clear that many important components of the biofilm arise, not from microorganisms, but rather from host origin in the absence of detectable infection. Moreover, such "endogenous" biofilm production can result in extensive accumulation of catheter-associated matrix.