Blood samples from normal adults and from members of seven families with the Swiss type of hereditary persistence of fetal hemoglobin (HPFH) from Yugoslavia were analyzed for their fetal hemoglobin (Hb F) and G gamma levels, while haplotyping defined the chromosomes at eight or nine polymorphic restriction sites. The data indicate that Swiss-HPFH, characterized by slightly elevated Hb F and G gamma levels and no recognizable hematological abnormality, is associated with a chromosome whose restriction enzyme haplotype is identical to the no. 3 (Senegal) haplotype found in black sickle cell (SS) patients. Many adults with this chromosome have high G gamma but normal Hb F levels. It is suggested that the Swiss-HPFH phenotype results from the action of more than one factor; one is linked to the beta-globin gene cluster and causes high G gamma values, while others result in an increased Hb F production and are perhaps of different origins.