Hematological and hemoglobin composition data are presented for seven Arabian SS patients with mild disease and with high Hb F levels varying between 21 and 34%. Four patients were homozygous for aβSchromosome with a specific haplotype (#31). The data for these four patients were similar to those for three other SS patients (and for five patients reported earlier, Ref. 2) who were heterozygous for the sameβSchromosome (#31) and for aβSchromosome with another haplotype (mainly #19). These data offer additional evidence indicating that the increasedγchain production is specific for theβSchromosome with haplotype #31. The similarities in hematological data and Hb F levels between these two groups of SS patients and the normal Hb F value in Hb S heterozygotes withβSchromosome (#31) support the suggestion that the increased Hb F production mainly occurs in response to the anemia of the sickle cell disease.