Partial or complete obstruction was diagnosed by means of sonography with water filling (hydrosonography) in 48 patients with rectal or colonic cancer. The procedure appeared highly effective in detecting pathological lesions larger than 1.0 cm. It may be useful under urgent surgical conditions as a method of choice in diagnosing the nature of lesion, size, endophytic growth or grade of invasion. Hydrosonography using retrograde contrast filling may considerably enhance the potential of standard and transabdominal scanning.