A test group of 153 children with summer diarrhea were compared with a matched contratest group of 100 children without diarrhea as to the results of bacteriological and virological examination of rectal swabs. In addition, acute and convalescent phase serums were obtained from many of the patients. The number of enteroviruses recovered from the rectal swabs of the test group was 2.5 times the number recovered from the contratest group. Viruses of the ECHO group were recovered six times more often from the children with diarrhea, and it is only to these viruses that a statistically significant etiological role could be assigned. Other important data as to the etiological significance and interrelations of enteropathogenic viruses and bacteria were obtained. It appeared that summer diarrhea of very young children is not an entity but rather a consequence of transitory infections with a large variety of bacterial and viral pathogens.