We evaluted measurement of urinary 4-hydroxyphenyl acetic acid as a potential screening method for small-bowel disease and bacterial overgrowth syndromes in 360 unselected acutely ill infants and children. Control data were obtained on 120 healthy children, ages 1.5 to 15 years, from a general medical practice, 48 healthy infants, ages one to five years, from local day nurseries, and 150 healthy babies, ages less than one to eight days. Comparative data were from 300 acutely ill hospitalized babies and children, ranging in age from less than one day to 15 years and without clinical evidence for small-bowel disease and bacterial overgrowth syndrome. No false-negative results and only 2% false-positive results were observed. Among the 10 patients whose urinary excretion of the analyte was considered to be abnormal were patients with Giardia lamblia infestation, ileal resection with blind loop, and other diseases of the small intestine associated with bacterial overgrowth. We conclude that measurement of 4-hydroxyphenylacetic acid excretion is useful in screening for such diseases.