A total of 111 patients referred with a diagnosis of suspected "appendicitis" were entered into a prospective study. The surgeon and radiologist in charge of ultrasonography made separate diagnoses, and their findings were then combined and discussed as indications for surgery. Clinically, a history of pain migration proved to be reliable (p < 0.0001) as a diagnostic indicator, in contrast to nausea and initial irregularity of bowels. The duration of symptoms was significantly shorter in patients with proved appendicitis than among patients with negative findings (median 24 hours compared with 41 hours, p < 0.04). Among patients with perforated appendicitis, the symptomatic history was prolonged (not significantly) by 3 hours. Peritoneal signs such as pain on percussion, rebound tenderness, guarding, and a leukocytosis of more than 13,000/mm3 were indicative of appendicitis (p = 0.0001 for each sign). Lively bowel sounds excluded the possibility of appendicitis (p = 0.001). Scanty bowel sounds, rectal tenderness, axillorectal temperature difference, and a left shift in leukocytes were of no diagnostic significance. The doctor's "clinical experience" is significant at the level of p < 0.03. On ultrasonography, the following signs were indicative of appendicitis: periappendicular infiltration (p = 0.0003), a visible "cockade," and an appendix larger than 12 mm in diameter (p = 0.04). For 75% of the patients the surgeon was sure of his own clinical diagnosis and did not allow himself to be influenced by the sonographic findings. In 12% of doubtful cases ultrasonographic results decisively favored operation, and in 4.5% (n = 5) it prevented an unnecessary laparotomy in the presence of positive clinical symptoms.(ABSTRACT TRUNCATED AT 250 WORDS)