The frequency of Campylobacter jejuni infection in children under 4 years of age, with and without diarrhea, attending the Hospital Infantil de Mexico was studied. Campylobacter was cultured from rectal swabs in 23 (8.7%) of 265 children with diarrhea, with the highest isolation rate in the 7- to 12-month age group, and in 2 (4%) of 54 children without diarrhea. From 5 (22%) of the children infected with Campylobacter, Salmonella or Shigella sp. were simultaneously cultured. Major clinical features of the patients with Campylobacter were similar to those observed in children with nontyphoidal salmonellosis. The isolation rate of Campylobacter was particularly high during the warm season. From these data it is difficult to establish a causative relationship between the presence of Campylobacter and diarrheal disease.