This study of Iranian families assessed the usefulness of carrier detection of phenylketonuria by variable number tandem-repeat (VNTR) polymorphism analysis. We studied 171 people (45 unrelated PKU subjects, and their parents and unaffected siblings). Of 342 chromosomes (131 non-PKU and 211 PKU), 5 VNTR alleles were identified. This VNTR system would yield a polymorphism information content of 66%, comparable to that in Europeans and higher than in Chinese. Carrier detection by segregation analysis of VNTR was informative in 89.5% of siblings. We conclude that this polymorphism is highly informative in carrier detection of PKU in the Iranian population.