Analysis of urinary organic acids in patients admitted for screening for inborn errors of metabolism incidentally revealed the presence of abnormal amounts of 4-hydroxyphenyllactate (4-HPLA) and phenyllactate (PLA). These compounds are found in tyrosinaemia and phenylketonuria but in our patients such disorders could not be established. By means of configuration analysis it was shown that these 2-hydroxyacids consisted partly of the D-enantiomers, pointing to a bacterial origin. Endogenously formed urinary 2-hydroxyacids in tyrosinaemia or phenylketonuria consisted of only the L-enantiomers. Furthermore, the urine of a patient with an established short bowel syndrome contained a wide variety of bacterial amino acid metabolites, including 2-hydroxyisocaproic acid (2-HICA). In this case 2-HICA occurred predominantly in the D-form whereas in the urine of a patient with maple syrup urine disease this compound appeared to have the L-configuration.