The symptoms of right-sided renal colic mimic sometimes acute appendicitis. A prospective comparative study of 188 patients with ureteral stone and 188 patients with acute appendicitis was performed to evaluate the features of differential diagnosis. Appendicitis caused more often nausea (81 vs 11%), fever and localized pain in the McBurney (97 vs 59%) than renal colic. The patients with ureteral stone had tenderness in 16% in the right lower quadrant. The mean values of C-reactive protein (41 mg/l) and blood leukocytes (14 x 10(9)/l) were elevated in appendicitis, but not in renal colic (14 mg/l and 10 x 10(9)/l). Urinanalysis revealed red cells in 92% of ureteral stones compared with 26% in appendicitis. Only one of 188 patients with apppendicitis was first misdiagnosed to have renal colic. A mistake of appendicitis for ureteral stone is clinically rare occurring only once or twice per year in the hospital where 700-800 emergency appendectomies are annually performed.