Diagnosing appendicitis may be difficult. We report the use of a new technetium-99m-albumin colloid white blood cell (TAC-WBC) scan in the evaluation of appendicitis. In a synthesis requiring 75 minutes, autologous neutrophils and macrophages from 40 mL of whole blood were labelled with technetium-99m-albumin colloid and administered to 100 patients with possible appendicitis. The entire process, from labelling the cells to completion of the scan took a maximum of 5 1/4 hours. Two patients had second scans on separate hospitalizations. Twenty-six patients had appendicitis; 12 had perforations, five of whom had an abscess. Eighty-five scans were read as either positive or negative for appendiceal pathology with a sensitivity of 89%, a specificity of 92%, and an accuracy of 92% in diagnosing appendicitis. Seventeen scans were indeterminant; eight of these patients had appendicitis. The value of the TAC-WBC scan in the evaluation of appendicitis lies in its ability to be used emergently, its high negative predictive value for men and women (NPV = 97%), and its high positive predictive value for men (PPV = 93%). At present, the scan does not appear to be reliable in diagnosing appendicitis in women (PPV = 43%). It is most useful in those patients in whom diagnosis is uncertain, and should not be used in patients with clear-cut appendicitis in whom its use will delay definitive surgical care.