The importance of sonography for the early detection and follow-up of tunnel infections in peritoneal dialysis patients is well documented, whereas other indications are less clear. We investigated indications and outcome of 738 ultrasound examinations of the peritoneal dialysis catheter tunnel. Indications for tunnel sonography included routine screening (27%), exit-site infection without peritonitis (24.1%), follow-up of tunnel infection (29.5%), clarification of questionable results (7.5%), pain in the course of the catheter tunnel (1.8%), peritonitis without (5.3%) and with (2.0%) exit-site infection, search for foci (2.2%), and recurrent peritonitis (0.7%). No positive sonographic results were obtained during routine screening or in patients with fever or elevated C-reactive protein levels showing no clinical signs of exit-site infection. Sonographic examinations were positive in 1 of 13 patients with pain in the course of the catheter tunnel, in 1 of 39 cases of peritonitis not associated with exit-site infection, in 12 of 15 patients with peritonitis and simultaneous exit-site infection, and in 2 of 5 patients with recurrent peritonitis. Questionable results were detected in 15 of 178 patients with exit-site infection, in 15 of 199 routine examinations, in 2 of 16 examinations of patients with elevated C-reactive protein levels or fever, and in 2 of 15 cases of peritonitis and simultaneous exit-site infection. All but two of these questionable results had to be rated as negative during further follow-up. We conclude that tunnel sonography is indicated in patients with exit-site infection (including cases with simultaneous peritonitis), for follow-up of tunnel infections, and for estimating the prognosis of these infections. Furthermore, tunnel sonography should be performed in patients with recurrent peritonitis. Tunnel sonography is not indicated for routine screening, search for foci, in cases of peritonitis without exit-site infection, or in patients with pain in the course of the catheter tunnel showing no other clinical signs of exit-site infection.