The total leucocyte count (TLC) and neutrophil percentage (NP) were studied prospectively in 1032 patients who underwent appendicectomy for suspected acute appendicitis. On histopathological study of the resected appendices, 97 patients had normal appendices, and 935 patients had acutely inflamed, gangrenous or perforated appendicitis. Statistically more patients with appendicitis had either raised TLC or raised NP compared with patients with normal appendices (P less than 0.001), and also compared with 357 patients who were admitted with right lower quadrant abdominal pain but were not subjected to operation (P less than 0.001). The sensitivity and specificity of raised TLC in diagnosing appendicitis for patients who underwent appendicectomy were 81.4% and 77.3%, respectively. Sensitivity increased when either raised TLC or raised NP were used, but specificity decreased. In contrast, sensitivity decreased when both raised TLC and raised NP were used together, but specificity increased. Raised TLC, preferably combined with raised NP, is a useful diagnostic aid in acute appendicitis. The TLC and NP, however, should only be interpreted in the light of physical findings in patients with suspected appendicitis because these blood tests have false positive and false negative results.