Investigations by scanning electron microscopy into changes of surface morphology of small bowel ***nucosa in children with chronic nonspecific diarrhea are reported. The study population comprised 56 patients, ranging in age from 5 months to 7 years: 65% were between 10 and 28 months old, and 64% of the patients were boys. The major findings were: microorganisms on he mucosal surface: excessive extrusion of cell cytoplasm and of enterocytes (cell shedding): presence of excessive mucus on the mucosal surface: damage to the brush border; and partial villous atrophy. The latter lesion was found in only four patients. All these changes are considered pathologic and, for the most part, are presumed to be due to the presence of antigens, in particular, nicroorganisms. A depression of disaccharidase activities was encountered in 64% of the patients, but prevalence was without regard to age. Most common was a combined depression of lactase, sucrase, and maltase, as well as an isolated depression of lactase. The possibility has to be considered that enteroadherent microorganisms which are usually not considered pathogenic, and microorganisms such asMycoplasma, may emerge as intestinal pathogens in susceptible children. It is feasible that genetic traits of the host and environmental factors facilitate adherence and colonization of the small bowel mucosa which, in turn, produces chronic diarrhea. Further studies are needed to confirm the preliminary information contained in this report.