Deficient aromatic hydroxylation of S-mephenytoin was observed in an index subject during a kinetic study of stereoselective metabolism of mephenytoin. A genetic basis for this defect was suggested by decreased urinary recovery of 3-methyl-5-(4-hydroxyphenyl)-5-ethylhydantoin (4-OH-M) in the 24 hr after oral racemic mephenytoin in two brothers of the propositus. The parents and a third brother had urinary recoveries of 4-OH-M of the same order as in a group of 20 normal subjects. The kinetic implications of this defect were studied in the index subject and compared with four normal subjects after a single oral dose of differentially radiolabeled pseudoracemic mephenytoin (5 microCi of 14C-S-mephenytoin, 45 microCi of H3-R-mephenytoin, and 11.5 mumol/kg of both S- and R-mephenytoin) followed by single oral doses of 1.4 mmol of unlabeled racemic mephenytoin daily the next 4 days. In normal subjects, there was substrate stereoselective metabolism with the S-enantiomer rapidly excreted as 4-OH-M and the R-enantiomer slowly excreted as 5-phenyl-5-ethylhydantoin (PEH). Stereoselective metabolism persisted during repeated dosing. In the hydroxylation-deficient subject, there was no evidence of stereoselective metabolism, recovery of 4-OH-M was low, and both enantiomers were slowly excreted, predominantly as PEH. Plasma PEH concentrations and urinary PEH excretion rates were approximately twice that in normal subjects. Thus a genetic deficiency in ability to hydroxylate S-mephenytoin results in the S-enantiomer metabolization by the alternate route of demethylation to PEH that cumulates, thereby, in comparison to the normal, effectively doubling the dose of total hydantoin.