The incidence of phenylketonuria (PKU) in Finland is extremely low, probably below 1 in 100,000. We describe the mutations and haplotypes in all four presently known patients. Mutation R408W was found on four mutant chromosomes (all haplotype 2), and IVS7nt1, R261Q, and IVS2nt1 were each found on a single chromosome. No mutation was found on the remaining chromosome. These findings support a pronounced negative founder effect as the cause of the low incidence of PKU in Finland, and are consistent with existing data regarding the European and Baltic origin of Finnish genes.