We present a case of twin Hispanic male infants fed with cow's milk formula who presented at 3 weeks of life with nonbilious, nonbloody vomiting and diarrhea. Laboratory evaluation revealed leukocytosis, acidosis, and methemoglobinemia. Sepsis evaluation was negative. Although they recovered quickly with i.v. fluids, symptoms recurred again with ingestion of soy formula. An underlying diagnosis was sought that could explain their symptoms.