A 10‐year old female on phenytoin therapy developed a rash and lymphadenopathy. H.p.l.c. assays of urinary metabolites indicated no differences in stereoselective metabolism of phenytoin to phenolic and dihydrodiol metabolites as compared with volunteers given the drug or with pediatric patients without adverse reactions. This suggests that no obvious difference in stereoselective metabolism of phenytoin to potentially toxic arene oxides exists between this patient and other patients on phenytoin. Our results are consistent with the hypothesis that differences in peripheral detoxication of phenytoin arene oxides and not differences in hepatic metabolism of phenytoin may be responsible for such adverse reactions.