A mutation at codon 974 of the dihydropyrimidine dehydrogenase (DPD) gene was previously described in a cancer patient with undetectable DPD enzyme activity who experienced severe toxicity when treated with 5-fluorouracil. We have studied the frequency of this mutation in 29 Scottish subjects with low DPD enzyme activity and in 274 American subjects. We detected no mutations in the 606 alleles studied and conclude that mutations at codon 974 are a rare event.