To examine the factors that may influence the outcome of infections by Shigella spp. and Campylobacter jejuni we followed for 24 consecutive months 321 rural Guatemala children 0 to 35 months old. Home visits were made to determine child morbidity patterns with emphasis on diarrhea and dysentery. Fecal samples for microbiologic studies were obtained from the participants when they were ill and during healthy periods. Shigella spp. were isolated from 9.8 and 4.0% of ill and healthy children, respectively; the figures for C. jejuni were 12.1% and 8.1%. Shigella flexneri 1, 2 and 6 and Shigella sonnei accounted for 70% of all Shigella isolates. Twenty-four percent of Shigella spp. and 7% of C. jejuni infections resulted in dysentery. Shigella dysenteriae and Shigella flexneri were more likely to induce dysentery than the other species. The incidence of dysentery was 0.84 of 100 child weeks. Age, gender, nutritional status and feeding habits of the children did not affect the outcome of Shigella infection. Fat consumption favored the development of dysentery caused by C. jejuni. The development of dysentery seems to be associated with microbial factors and not with host variables, although specific Shigella serotype protection against symptomatic infection may be functional for prolonged periods after natural exposure.