Fecal specimens from 101 patients with diarrhea were cultured and also examined with methylene blue for leukocytes. Thirty-six patients had leukocytes in their stools and 29 had culture-proven shigellosis. The sensitivity of fecal leukocytes in shigellosis was 95% (19/20) when cup specimens were obtained, and 44% (4/9) when swab or diaper specimens were examined. Only 45% of the patients with shigellosis who provided cup specimens had grossly bloody dysentery. Twelve other patients had fecal leukocytes but no demonstrable invasive bacterial pathogens. Methylene blue examination was useful in identifying motile trophozoites of Giardia lamblia and eggs or larvae of other heavy intestinal parasitic infections. Among patients with naturally-acquired acute diarrhea, methylene blue examination of stools for leukocytes is much more sensitive than examination for blood in predicting a positive culture for Shigella spp. It is also of value in detecting parasites.