PURPOSE: In our center since 2001, follow-up examination has included three-dimensional endosonography in all patients with suspicion of local recurrence of anal cancer. This study was designed to investigate whether three-dimensional endosonography surpassed two-dimensional endosonography as a diagnostic tool for patients with suspected local recurrence. METHODS: This prospective study included 38 consecutive patients who have had anal carcinoma and were investigated using three-dimensional endosonography in combination with anoscopy and digital rectal examination at Rigshospitalet from July 2001 to January 2005 under suspicion of local recurrence. All endosonographic examinations-two-dimensional, three-dimensional, and three-dimensional in combination with anoscopy and digital rectal examination-were evaluated by blinded observers. The observers scored each examination according to a five-point scale in which a score from 1 to 3 was regarded as benign endosonographic findings and a score from 4 to 5 was regarded as malignant endosonographic findings. The endosonographic diagnosis for each examination was compared with histologic evaluation or when no biopsy had been taken with a follow-up period of at least six months. If a patient showed no signs of local recurrence in the follow-up period, no local recurrence was considered to be present at the time of the investigation. RESULTS: The sensitivity was 1.0 for three-dimensional endosonography in combination with palpation, 0.86 for three-dimensional endosonography alone, and 0.57 for two-dimensional endosonography. The differences between two-dimensional endosonography and three-dimensional endosonography alone as well as two-dimensional endosonography and three-dimensional endosonography + anoscopy and digital rectal examination both reached significance with P values <0.05. CONCLUSIONS: This study indicates that three-dimensional endosonography surpasses two-dimensional endosonography in the evaluation of patients with suspicion of local recurrence of anal cancer especially in combination with anoscopy and digital rectal examination.