The possibilities of using ultrasound (US) diagnosis and color Doppler flow mapping both to detect rectal carcinoma (RC) and to specify its extent were determined. The investigation was conducted comparing the results of US and histological studies of the extent and histological forms of RC in 140 patients. The absence of differentiation of rectal wall layers was of value as being an indicator of malignant transformation in 98.9% of cases. The majority (96.1%) of malignant processes had a statistically significantly high blood flow resistance index (more than 0.5). US study was shown to be a valid and reliable diagnostic test to detect primary rectal tumor, to estimate its spread into the wall of the organ, and to identify metastatic changes in lymph nodes and parenchymatous organs.