For the purpose of better understanding the epidemiology of acute and persistent diarrhea, 130 infants of a marginal urban area in Guatemala City were studied. The subjects were kept under surveillance by weekly home visits, for periods that varied from three to nine months. The diarrhea episodes were detected and microbiological studies were done in fecal material. Additionally, the children were weighed and measured to determine their nutritional status. The infants suffered, on the average, 5.2 episodes per child annually; 9.4% of all the episodes lasted at least two weeks. The children who were less than six months old had more episodes of persistent diarrhea (0.052/child-month) than the older ones (0.017/child-month), with previous diarrhea morbidity and number of infecting enteropathogens being important factors. Furthermore, a child who had already suffered an episode of persistent diarrhea had a higher probability (relative risk = 2.2) of developing an additional one. Adherent E. coli, Cryptosporidium, toxigenic E. coli and Campylobacter jejuni are the pathogens more commonly associated with persistent diarrhea. Diarrheal illnesses have a deleterious effect on nutritional status, especially persistent episodes, which interfere with gain in weight and length of the children.