A 7-week-old infant with methemoglobinemia, hemolytic anemia, and inadequate weight gain was found to have aCampylobacter jejunigastrointestinal tract infection. Known etiologies of methemoglobinemia such as oxidative drug exposure, deficiency of NADH-methemoglobin reductase, and hemoglobin M disorder were excluded. The patient had a twin brother (probably identical) who had neither methemoglobinemia nor stool cultures positive forC. jejuni.The twin essentially served as an experimental control, making other environmental or genetic causes, of methemoglobinemia unlikely in the patient. Both the methemoglobinemia and theC. jejuniinfection responded to adequate treatment with erythromycin. The association of aC. jejuniinfection with methemoglobinemia is discussed in light of previous associations of enteritis and methemoglobinemia in infants.