Microscopic stool examination can distinguish inflammatory from noninflammatory diarrheas. The modified guaiac test was shown to have good correlation to stool microscopy. In a prospective study we evaluated the diagnostic accuracy of a modified guaiac test (Colo-Rectal-Test, Roche) and of an immunological test for fecal haemoglobin (Colo-Immun-Test, Roche) in relation to the diarrheal pathogens identified and compared it with the stool microscopy. In 304 patients, clinical presentation, stool microscopy, stool culture, and modified guaiac test were recorded. Sensitivity of the guaiac test was 69% as compared to 63-67% for the stool microscopy. Specificity could be improved by 10-15% using an immunological test to exclude false-positive guaiac reactions. A modified guaiac test can replace microscopic stool examination to distinguish between inflammatory and non-inflammatory diarrhea. Immunological testing for occult blood can improve the specificity of the guaiac test, but is too elaborate to serve as a screening test. The modified guaiac test can easily be handled by community health workers and could be important in the diagnostic work-up for acute infectious diarrhea.