To　avoid　or　reduce　the　X-ray　exposure　in　endoscopic　examinations　and　therapy　as　an　alternative　to　the　conventional　two-dimensional　X-ray　fluoroscopy　we　are　developing　an　intrabody　navigation　system　that　can　directly　measure　and　visualize　the　three-dimensional　(3-D)　position　of　the　tip　and　the　trace　of　an　ultrasound　endoscope.　The　proposed　system　can　identify　the　3-D　location　and　direction　of　the　endoscope　probe　inserted　into　the　body　to　furnish　endoscopic　images.　A　marker　transducer(s)　placed　on　the　surface　of　the　body　transmits　ultrasound　pulses,　which　are　visualized　as　a　marker　synchronized　to　the　scanning　of　the　endoscope.　The　position　(direction　and　distance　of　the　marker　transducer(s)　outside　the　body　relative　to　the　scanning　probe　inside　the　body)　of　the　marker　is　detected　and　measured　in　the　scanned　image　of　the　ultrasound　endoscope.　Further,　an　optical　localizer　locates　the　marker　transducer(s)　with　six　degrees　of　freedom.　Thus,　the　proposed　method　performs　inside-body　3-D　localization　by　utilizing　the　inherent　image　reconstruction　function　of　the　ultrasound　endoscope,　and　is　able　to　be　used　with　currently　available　commercial　ultrasound　image　scanners.　The　system　may　be　envisaged　as　a　kind　of　global　positioning　system　for　intrabody　navigation.