Preoperative total and differential leucocyte counts were made in 225 children with acute appendicitis, 50 children with a normal appendix and 100 children with acute abdominal pain which resolved without any therapy. When adjusted to age, neutrophilia in 96 per cent, 30 per cent and 32 per cent and leucocytosis in 41.8 per cent, 4 per cent and 3 per cent was noted in the respective groups, the levels being significantly higher in the acute appendicitis group than in the other two groups. Among children with appendicitis, infections were observed in 62.8 per cent of the group with leucocytosis and in 13.7 per cent of the group with a normal leucocyte count; the difference is statistically significant. It is suggested that in acute appendicitis neutrophilia supports the diagnosis and leucocytosis indicates the prognosis. The rate of infection increases as the total leucocyte count rises above the upper normal limit. The difference of opinions on the significance of leucocyte counts in acute appendicitis is discussed.