Eleven cases of pancreatic cystadenocarcinoma were studied with respect to the differential diagnosis from that of pancreatic pseudocysts, using diagnostic ultrasound as the initial investigative technique. Most of the cases presented an ultrasonic picture that correlated well with the gross pathology, but this correlation was not reliable enough to be considered characteristic in differentiating such masses from pseudocysts. The authors divide the cystadenocarcinomas of the pancreas into four echographic classes. In all but one of the classes, diagnostic ultrasound was insufficient by itself to render an accurate diagnosis. In such cases other techniques, i.e. CT, tissue biopsy, and especially angiography, were required for a reliable diagnosis.