Relationship between baseline (A) and 2-year (B) HbF levels and F-cell counts. HbF was measured by alkali denaturation and F cells by flow cytometry on samples obtained at baseline and at the conclusion of 2 years of HU therapy. (A) Relationship at baseline and (B) at 2 years of follow-up. In (B), data were pooled from patients assigned to HU and those assigned to placebo. See text for explanation.