The extent of involvement of the subcutaneous Tenckhoff catheter tract in CAPD peritonitis and catheter-related infections is of major therapeutic importance. By definition, both peritonitis and exit-site infections do not involve the catheter tract. However, diagnosis of these infections as well as the more sinister tunnel infection is based mainly on clinical signs. We examined the usefulness of ultrasound examination (US) of the catheter tract in delineating catheter-related (exit-site and tunnel) infections, and their relationship to each other and to peritonitis. CAPD patients with no evidence of peritonitis or catheter-related infections for 6 months prior to examination served as controls. US were performed by one of two experienced radiologists using the Acuson 128XP/10 scanner with a 7-MHz linear transducer. A positive US was defined as an area of hypoechogenicity (indicative of fluid collection) >2 mm in width along any portion of the catheter tract. Findings were localized into segments(S) as follows: S1, limited to external cuff; S2, intercuff segment adjacent to the external cuff; S3, intercuff segment adjacent to the internal cuff; S4, limited to the internal cuff; and S5, involvement extending throughout the catheter tract. Between March 1993 and January 1995, 39 CAPD patients, all with a double-cuff straight Tenckhoff catheter with the exit site situated above the point of entry into the peritoneum were studied. A total of 56 US were performed divided among 26 episodes of peritonitis, four tunnel infections, 13 exit-site infections,and 13 controls. There were 30 positive US distributed among 16 peritonitis, four tunnel, eight exit site infections and two control patients. The two positive controls went on to develop peritonitis within 1 month of the US. The majority of the US findings (13/16 in episodes of peritonitis and 5/8 exit site infections were localized to segment 4, that is, to the internal cuff region. Apart from a significant increase in width in all infected segments versus a normal tunnel, no differences in size were seen between peritonitis, exit-site, or tunnel infections, nor were there any differences in size and localization in these infections when comparing the offending organism (Gram-positive, negative, or culture negative). We conclude that peritonitis and exit-site infections are frequently accompanied by involvement of the catheter tract. The localization of infection to the internal cuff region in cases of exit-site infection probably occurred as a result of downward migration along the catheter tract. This supports the notion that ideally the exit site should be pointing caudally or that the peritoneal catheter have a swan-neck configuration. With regard to peritonitis, infection within the peritoneal cavity appears to extend and involve the internal cuff region. Thus both the internal and external cuffs do not seem to pose an effective barrier against the spread of infection.. Based on our data, we recommend that US be performed as a routine investigation in all cases of exit-site infection and in cases of refractory or relapsing peritonitis.