A 57 year old woman living independently in the community presented with four years of progressive spastic paraparesis and dementia. An extensive evaluation for the usual causes of these difficulties was unrevealing, but her serum phenylalanine concentration was markedly elevated and genetic analysis demonstrated mutations in the phenylalanine hydroxylase gene consistent with classic phenylketonuria. A protein restricted diet was associated with improvement in her condition. Although untreated phenylketonuria is typically associated with severe neurological dysfunction beginning in early childhood, this case shows that disability may be delayed until adulthood.