In a prospective study, we investigated the value of high resolution sonography for the imaging diagnosis of colonic diverticulitis. A total of 161 patients with abdominal complaints on account of which acute diverticulitis entered into differential diagnosis were included in the study. 74 of these finally had proven colonic diverticulitis (prevalence, 46%). The results of intestinal sonography and the working diagnoses on admission were evaluated on the basis of final diagnoses. According to history, symptoms, and clinical findings on admission, the working diagnosis of diverticulitis was graded as being "highly suspect" in 26 (35.1%) of these 74 diverticulitis patients, as being "possible/equivocal" in 36 (48.6%), and as being "very unlikely" in in 12 (16.2%). In 7 (9.5%) patients with diverticulitis, the diagnosis was clinically unsuspected and primarily established by ultrasound. Overall accuracy, sensitivity, and specificity of intestinal sonography with regard to the diagnosis of diverticulitis were 97.5%, 98.6%, and 96.5%, respectively. There were three false positive ultrasonic diagnoses of diverticulitis which were made in two patients with perforated carcinoma of the sigmoid and in one case of sigmoid volvulus (positive predictive value, 96%). Ultrasound failed to visualize sigmoid diverticulitis with perforation in one patient (negative predictive value, 98%). In 19 out of 20 patients with abdominal abscesses (prevalence, 12.4%), the diagnoses were established by means of sonography. Nine of these patients with abscesses initially were treated by ultrasound guided percutaneous drainage enabling elective surgery. In 26 (70%) of 36 patients with morphological disorders other than diverticulitis, ultrasound findings substantially added for the definite diagnoses.(ABSTRACT TRUNCATED AT 250 WORDS)