1.1. Objectives: Study of the sensitivity of Hydrocolonic Sonography (HS) in the detection of colonic lesions and how hydrocolonic sonography with echographic contrast agent (HSEC) can improve this technique. Methods:A prospective study of 73 patients (65 by HS and 8 by HSEC). Sixty-five HS examinations were made prior to optical colonoscopy (OC). The results obtained by HS were compared with those of OC. In the 8 HSEC studies no cleaning preparation was used and all patients were awaiting surgery for colon tumours. An anatomopathological study of the lesions was carried out. Statistical significance was tested using ANOVA. Results:In lesions of ≤ 5 mm, the HS had a sensitivity of 15.9% and a specificity of 100%; for lesions of 6-10 mm, the sensitivity was 70 % with a specificity of 96.1%. In lesions measuring over 10 mm the sensitivity was 93% and the specificity was 95.1%. In adenocarcinomas both parameters reached 100%. HS did not detect rectal lesions or flat lesions. All lesions studied by HSEC were enhanced, 88.9% were completely visualized, while with HS only 33.3% were